Except that Meghan's brother-in-law isn't just anybody. (Not to mention her father-in-law.)
William could very well become king within the next ten years, certainly the heir to an elderly king, and then it does become a matter for William, whether he would like to interfere or not.
So should the family continue to make a spectacle of themselves (they'll certainly try!) the Monarch, be that QEII, Charles or William may very well decide to step in and simply bar certain family members from attending anything remotely royal, let alone be let in at the palaces or in a more extreme case simply direct H&M to sever the connection to certain family members. - That could also save Meghan in particular considerable embarrassment, because then it's the Monarch, not her, who is cutting them off.
I would not be the slightest bit surprised if a family member is suddenly standing suitcase in hand outside Kensington Palace - with the press being notified of course. If H&M turn that person away or even put them in a hotel, then we have the big soap opera about "cruel, heartless Meghan" turning away her family to let them fend for themselves on the streets, selling matches... Complete with a chorus singing: Bou hou, bou hou...