Crazy Ex, Extreme Awkwardness, Bollywood-esque plotline, Need Input!

[Indus18]

My ex treated me horribly toward the end of our relationship (first love, 4 year relationship) and left me feeling extremely used, betrayed, and bitter after it ended. I finally started dating again about a year ago, but haven't been in a serious relationship since, and have actually stopped dating recently while I try to get my life back on the track I want.

Although it's been 2 years since our break-up, she still tries to contact me obsessively every couple months. (I'm talking numerous e-mails, phone calls, voice mails, and texts until I respond.) She has a lot of problems going on in her life (problems whcih I used to make my own, and which weighed on me heavily when we were dating) and in general, I'm incapable of being truly mean, particularly to someone going through a hard time, so it takes a long time for me to get her to promise to leave me alone, and the cycle restarts again a few months later. She claims she wants to be friends, but every time she guilts me into talking to her again, she always ends up throwing in these sexual, flirtatious messages and then proceeding to play innocent. It makes me very uncomfortable, because the last year of our relationship, we were on-again/off-again too many times to count, and I don't want to get anywhere near that destructive cycle again. (We've been truly broken up for about a year and a half now)

Now to the crux of the story. She was manageable while she was on the West Coast at college, as my home is in the midwest, and I go to school near there. However, I met this ex of mine through my older sister, who was dating her older brother. (Yes, it's pseudo-incestuous, but the heart wants what it wants, I guess) She and I dated for 4 years, my sister adn her brother ( both 6 years older than both me and my ex) dated for 6 years. They've just recently ended an engagement and remain close friends. Meanwhile, my ex and my sister's ex, have both been disowned by their psycho, abusive parents.

Thanksgiving is coming up, and I'm driving home from school for break, and my sister is flying in too. However, this recent development wherein both my sister's ex, and his sister have been told that they are no longer welcome in their own home has made some waves. My sister invited her ex to our place for thanksgiving because she feels that he should be surrounded by a warm, family environment (and he has spent the last 5 thanksgivings with us anyways.) He didn't want his sister to be alone, so my sister and my mom have invited her too, without consulting me.

I love my family; my sister is my dearest friend, and my mom is one of the purest-hearted people I know, but I am so infuriated about this situation. Conversely, THEY'RE both upset at ME, tellign me I'm being immature by saying I don't want to come home for break anymore. I can't express to you how crazy this girl is. Once, after we broke up the first time, while we were both home for break (Her family lives about an hour and a half's drive from my family) she drove all the way to my house without my consent, came up to my room where I was studying, and proceeded to tell me she loved me and missed me while trying to coerce me into some... sexual activity. It was creepy. I don't want to get entangled in her problems, and I haven't had sex in 6 months, and I know she's going to be making advances every chance she gets, and I'm just too emotionally fragile of a guy, to let someone who has hurt me so badly back into my life. I'm going through an EXTREMELY hard time in my own life, and this is the last thing I need right now. Am I being crazy for refusing to brave 4 days of this bizarre "holiday with the exes" drama? I just know that a whole lot of memories I've worked hard to bury are going to resurface, and that this trip will test my kindness, patience, sexual restraint, emotions, and will try my bonds with my family as well.

Both exes have done horrible thigns to me and my sister, things about which my naive mother couldn't even think up in her nightmares. I woudl expect my mother not to know why I'm so vehemently against seeing my ex AND my sister's ex, but I am shocked my sister doesn't know better after the cheating, lying, and manipulation both of us endured. What should I do? Both my mom and sister are saying they won't speak to me if I don't show up, AND i don't want to appear as though my ex still has such a big effect on me, because in some weird way, that would drive her to contact me more. (Sounds crazy, but you don't know her.) Any advice?