I love this question. I'm going to break it down a bit.
Is being the monarch's in-law's, are they given any title just because your related? Is this how a commoner becomes upper class/entitled/Royal?
This just depends on the monarch, really. There's never really been established rules of who gets a peerage and who doesn't, at least when it extends to people who themselves aren't royals.
This isn't necessarily how someone, or a family, became aristocratic/noble, and certainly wasn't (in the British system at least) how they became royal. For the most part, until rather recently people who married into the English/British royal families have always been from at least upper class, if not aristocratic or royal backgrounds, prior to their marriages. In a few cases, women whose fathers, or families, didn't hold any titles married into the family, sometimes resulting in their families being given titles. It just depended on the monarch.
How commoners became nobles wasn't simply that they married into the royal family. Typically, honours were bestowed on people who had done a service to the monarch, monarchy, or government. Often it seems like these were granted in succession - typically, for awhile a number of fathers and sons would be knighted, then later one might be created a baronet, later on a descendant might be created an earl, then later another might become a duke.... It didn't always follow a set order and it seems like when the younger sons or grandsons of men who were peers were created peers themselves they didn't start back at zero.
So, consider the Spencer family. The first John Spencer was created a knight during the reign of Henry VIII, and a few generations of sons and grandsons were similarly knighted, before one such was created Baron Spencer of Wormleighton. Later a Baron Spencer was created Earl of Sunderland. When the grandson of an Earl of Sunderland was given a peerage it reflected the fact that he was descended from peers - first he was created Viscount Spencer, then later Earl Spencer (as well as secondary titles with both peerages).
When William becomes King, could he bestow a title on his in-laws just because?
This is a subject that can be debated a bit. As King, William will be the fount of all honours. Technically speaking, he could bestow a title on his father-in-law, or even brother-in-law, if he so wished (he could give his mother- or sister-in-law a title as well, but titles for women are considerably less common), but it would be going against the trend towards titles of the day (assuming that this continues). Currently, the only people to be granted hereditary peerages are royals, and life peerages are only granted on the advice of the government, typically after achieving something noteworthy. As such, I would debate whether or not William could create a peerage for any of the Middletons "just because." Even if he could, I think it would risk seriously offending some of his subjects.
What about when King Henry VIII married he had lots of in-laws. Did he bestow titles to them? One wife was made his sister? Was that a great honor back then? What did she get as a sister that as an ex wife she would not have gotten?
Henry VIII married six times. His first wife, Catherine of Aragon, was a princess in her own right and came from a family with their own titles. Henry didn't, to the best of my knowledge, bestow any English titles upon them.
His second wife was Anne Boleyn. Anne's father didn't inherit titles and it's very likely that the titles that he received were largely related to his daughter's relationship, but Thomas Boleyn was also involved in politics so at least some of the honours he received may have been because of his own merits. Anne's brother wasn't given his own peerage, being expected to inherit his father's titles, but he did use the title Viscount Rochford by courtesy, but his titles were forfeited when he was executed. Anne was also granted her own peerage, a Marquessate, prior to, but because of, her marriage.
His third wife was Jane Seymour. Jane's father, John, was knighted by Henry VII, initially for his involvement in the Cornish Rebellion of 1497. John died within a year of Jane's marriage and was never granted a peerage. Two of Jane's brothers, Edward and Thomas, were created peers, but I believe this was more because they were the uncles (and regents) of the King than because they were the brothers-in-law of the King - in the case of Edward, he was created Duke of Somerset in 1547, 10 years after his sister's death and the year his nephew became king. I believe Thomas became a Baron around the same time. Around the same time, Jane's youngest brother, Henry, was created a knight, but he never received any further titles (and unlike his elder brothers, was never executed when his ambition backfired, so he didn't necessarily lose out).
His fourth wife was Anne of Cleves. Anne was from a Continental Ruling family, and as such her family had no need of English titles. Anne was the wife who was created an honourary member of the family after the annulment of her marriage, and it did make a difference. Henry had only divorced one other wife at this point - Catherine - and she didn't exactly have a great post-marriage situation. His two other wives, at this point, didn't survive their marriages. In saying that she was to be treated as the King's sister, Henry basically assured that Anne was to be treated well during his reign, and even afterwards. She was essentially taken care of and remained connected to the court.
Wife five was Catherine Howard, whose father died in the year before she got married. Both of her brothers were courtiers during her marriage, but neither were created peers. Catherine's youngest brother was eventually knighted because of his services on the battle field, after Henry's death. Interestingly, this knighthood was granted by the Duke of Somerset, the eldest brother of wife number 3.
The last wife was Catherine Parr. Catherine's father was knighted in 1509 and died in 1517, well before his daughter's marriage to the King. Catherine's brother, William, was first created a Baron a few years before her marriage, then created an Earl after the marriage, then finally created a Marquess after Henry's death. I would assume that the initial peerage had nothing to do with Catherine, the second one was because of his position as the brother-in-law of the king, and the third one was because he was the King's uncle (in a round-about way). At the same time, though, William Parr was also involved in politics and the court.
What about other Royal Families?
I honestly don't know about other Royal Families, but I can comment a bit more on other British monarchs. Henry VIII may be the most famous in terms of his marriages, but he's not the only one to marry women from more common means.
Edward IV's wife, Elizabeth Woodville, was born to a commoner, Richard Woodville, later 1st Earl Rivers. Richard's peerage was very likely due to his daughter's marriage, as it was granted during Edward's reign and after the wedding.
James II's first wife, Anne Hyde, was also born to a commoner-turned-peer, Edward Hyde. Edward's rise to the peerage, however, wasn't really connected to his daughter's royal marriage so much as it was to Edward's continued support of the King through the Civil War, Exile, and later Restoration. All of Edward's peerages were granted to him by Charles II with a year of both the restoration and Anne's marriage, at a time when it was still considered likely that Charles would father legitimate children (Charles wasn't yet married at the time).
As for the Scots, David II married a woman whose father was just a knight, but I couldn't tell you why he wasn't granted further peerages (be it a deliberate omission on the part of David, or owing to the father's death, although I suspect the latter as it wasn't either's first marriage). Robert II's first father-in-law was only a knight, but Robert's first wife died before he became king. Mary of Scots' second husband, Henry Stuart, only held his titles by courtesy, but he predeceased his father, who did have his own, pre-existing peerage. All other Scottish monarchs married women whose fathers had titles of their own.