I can't see either of the York princesses wanting, or getting, Westminster Abbey. The expenses alone, especially for security, would be astronomical, considering that many of the BRF would be guests.
Whoever marries first will, IMHO, probably wed at St George's Chapel Windsor, but I doubt there would be full TV coverage. If Eugenie marries after her sister I can imagine her going for a low key wedding on one of the estates, Sandringham in the autumn or Balmoral in the summer. I don't think either girl will get full TV coverage for their weddings. They are minor royals, Andrew's influence within the family is fading and they are just not popular enough with the British public, for various reasons.
Why would George and Charlotte's existence impact on Harry's wedding? He is the last senior Royal of his generation to marry and is very popular with the British public. If he and his wife wanted to, and that's debatable, he could marry at Westminster Abbey, with George as pageboy and Charlotte, if she's old enough, as bridesmaid. If the couple choose St George's instead, that will be their choice.