My girlfriend is threatening to break up with me if I don't have with her fat sister help!?

My girlfriend Lena is seriously creeping me the hell out of late, she recently gave up being a Christian and embraced a lifestyle of hedonistic debauchery partying with her girl friends late into the night getting drunk and bringing girls home for us to double team together. Now as a guy I'm of course more than ok with THAT but here's the kicker as my title described. Her sister Lisa is at least by my estimates approaching the 400lb mark with no indication in sight that she's going to give up the Pizza Hut menu and Mcdonalds Quarter Pounders in favor of a disciplined diet and exercise regimen. Lisa predictably has been depressed for several years due to never having a boyfriend and fearing she will never lose her virginity. I know right? BIG surprise there. So my girlfriend suggested to me that because of all the she and I have had with other girls that she "hunted" specifically for me that I owe her a favor in return. She wants me to de-flower Lisa and tells me I could get drunk beforehand if that would make it easier. Now people Lisa even if she were in shape has a face that could break a mirror and make a baby cry. It took every ounce of my willpower not to throw up in my mouth a bit when Lena suggested this to me. Whats worse and what surprised me is that Lisa isn't offended by Lena's idea to help her out, infact she's all for it and has always told Lena that she envies her beauty and being able to have a boyfriend as "studly as I am" (her words) and that Lena owes her anyway since she used to do some of Lena's harder homework when they were both kids. So fellas the answer would seem obvious ditch the b...BUT...my god the with Lena is just so raw...so so...crazy...so hot. Plus her ability to get attractive girls for double team action is second to none in my experience...its just hard to let her go even with such a sickening bargaining chip to keep her love placed before me...so...What would you do?