Sorry, GEH, but I am not accepting that you should have to bow out of family parties or endure her comments. You're too good for that and deserve for your family to have. your. back. And her drinking and status as "family?" Nuh-uh. Not an excuse. FAMILY doesn't do this. FAMILY acts the part. She's just a nasty barely-relative-by-marriage.
I am *fuming* on your behalf. FUMING. What horrid, horrid, sorry excuse of a human being and I'm equally disgusted by your family not excluding her from gatherings or doing a better job of taking care of their own. Your Aunt Awesome needs to learn the word "no" when BitterHag invites herself over.
The good news, though--I admire that you have a spine with her. That is a tremendous first step! I think it should extend to your family, though. Can you sit down with your parents, aunt, etc and tell them about the abuse (yep this is abuse!) and ask them to help beandip? Explain that you love them, family gatherings mean everything to you, but it's unfair that you have to endure this garbage because of their inability to back you up. Because, yes, the onus is on them. Sometimes, one spine isn't enough; your family needs to grow one collectively. Turn this around on them to provide a welcoming environment for you. Why can they do it for her, and not for you?
I agree 100% with some of the the PPs. There is some kind of jealousy/projection. Maybe you're better educated, she thinks you make more money, are more well-traveled, etc. Maybe she's insecure about her "place" in the family or that one of her kids doesn't compare to you, in her mind. Either way, sounds like you have your head on straight and know that this has nothing to do with *you.*
If you choose to attend, I think you're allowed to measured retaliatory rudeness. I'm normally not in favor, but know what? If this is what it takes to change how her behavior affects you, well, so be it.
BitterHag: "Oh, Diane, looks like you've gained a few!"
GEH: "Gold Star for stating the obvious! This conversation is over." Lather, rinse, repeat.
BTW, in case it helps, I spent part of my teen years overweight, lost it in a healthy way, gained a lot of weight in college and am down 70 pounds after a few years of hard work. As such, I unfortunately have experience with people like this and an arsenal of snarky, clever retorts. But they shouldn't be necessary to share--recognize that your weight is something she perceives as your Achilles heel and the best way to deal with a bully is to take away their power. Step one: get your family to wash off the "Welcome" on their collective forehead and develop a plan to either back you up or to exclude her. Step two: Keep on standing up for yourself. That is the most important. Because you don't deserve this.