I am actually gonna say YTA.

• You have known her six months. Why would she pick you over a sister? Why would she add more people to her party because you feel left out?
• There is nothing weird about walking down an aisle with someone who isn't your wife. Again, you do not know the bride well enough to punt the other person out. And insisting that you be part of the wedding because feel entitled is... tacky.
• You have every right to decline doing favours for the bride and groom... but I don't actually think she is doing this to spite you... or whatever you think is happening. I think she is trying to include you because you seem offended you were left out.