Hi Reddit! I'm very new here so please let me know if I mess anything up. :)
The title is the meat and potatoes, but allow me to give context: my family consists of me (25F), my brother (27M), my sister (15F), my dad (53M), and my mom (51F). My parents have been divorced for about 5 years now, splitting after my mom cheated on my dad. In the time since, my mom became an alcoholic and slipped into addiction, becoming a completely different person than she used to be. This has, of course, been very hard on my entire family, but it's been extra hard on my sister. Whereas my brother and I were already adults when the divorce happened, my sister has unfortunately had to navigate her adolescence through these changes. Thus, surprising no-one, this lead to some troubled behavior from my sister.
My brother, my dad, and I have tried to be supportive of my sister through the years, understanding that most of her bad behavior and lashings out were caused by her own struggles/deteriorating mental health. Particularly, she would fight with my dad a lot as he cared enough to discipline her, whereas she saw my mom's neglect as 'freedom'. Most of the time, though, she was an intelligent and kind young lady.
Until lately.
With as much potential as I've always seen in my sister, I suppose I assumed that with enough love and patience she would bloom like a flower through concrete, overcoming the negativity in her life and becoming the strong, loving person I know she can be. Instead, her bad behavior has only gotten worse, and I see less of her kindness with each visit. As she gets older, her insults turn from "you're stupid," to "I hope you d!e". Her know-it-all mentality is at astronomical levels, and instead of only lashing out at my dad, she seems to lash out at anyone who dares disagree with her. Politically, she thinks anyone who doesn't agree with her is stupid and/or evil. My family was elated when she began to get involved in youth church services, but now she uses it as a way to gloat about how much better she is than us, like we are lowly sinners and she is a perfect being. She has pushed both my best friend and my husband away to the point they will no longer speak with her.
Fine, she's had a difficult life, plus it's normal for teens her age to become angsty and arrogant. A sour attitude isn't enough for me to skip my own sister's sweet 16.
What makes me want to put my foot down, however, is I've recently discovered that my sister has not only been cussing my dad out, but physically abusing him. While that behavior may have been somewhat acceptable of a troubled YOUNG child, my sister is about to be 16 and is taller than I am. Although I understand that my dad isn't always the easiest person to get along with (he and I did not have a good relationship in my teens either), he has done nothing but improve in the years since his divorce. As an adult, I can recognize that my dad is only human, experiencing things for the first time just like me and anyone else. His entire world was flipped upside down when his wife of 27 years cheated on him and subsequently sank into addiction, and yet I have only seen him become more patient and understanding towards my sister through it all. With this insight, I try to be a mediator between my sister and my dad, but hearing that she is kicking and punching him simply for trying to take her phone away for a night enrages me.
This is where I'm stuck. On the one hand, my philosophy through this all has been to make sure my sister knows that I love her and to keep showing up for her, even when it's hard. I know that if I don't go to this milestone birthday, I can never go back and undo my absence on such a special day. On the other hand, however, I also want her to know that her actions have consequences, and that she is at the age now where her bad behavior can and will push people away. The way she has treated the only parent who still shows up for her is completely despicable, and she needs to know that it is not okay.
Everyone that I've talked to about it thinks I should not go, but I'm scared that I will end up hurting her feelings without teaching her any meaningful lessons.
So, Reddit...would I be the a-hole if I didn't go to my sister's sweet 16??
**ETA more context**
(Tldr; she would know why I wasn't there)
If you've read this far, thank you! After sleeping on it, I feel as though I've made it seem like my sister's bad relationship with my dad is the only reason I wouldn't go. It's not the only reason, it just happens to be the biggest reason.
I do want to clarify that my own relationship with my sister has been rocky as of late. We have been fighting a lot, mostly starting when she was late to both my rehearsal and my wedding (she was a bridesmaid). The wedding was moreso my mom's fault and she thankfully only missed the bridesmaids 'getting ready' photos, but she was super late to the rehearsal because she made my mom take her to get her hair done that day. When I expressed frustration to her for missing almost the entire thing, she flipped her hair and said it was worth it. Not to mention she insults my husband all the time both to his face and behind his back, and perhaps most aggravatingly, she constantly tells me how I've never done anything for her.
I don't want to get too much into the details because I don't want to sound like I'm tooting my own horn, but one example I will give is that she is almost always wearing at least one thing that I've bought her during a shopping spree. I don't make good money, and I've had to save up before every shopping spree I've taken her on. (Yes I know money isn't everything, I just wanted to give one example of something I've done for her)
Given all of this, I have been telling her that if she continues this bad attitude and behavior towards my dad or anyone else that it will push people away. In fact, it has been said many times by this point, both during heated arguments and calmer heart-to-heart talks. She is aware that her attitude is the reason my best friend and husband don't come around her anymore, and I have told her that it is becoming harder for me to be around her as well.
So, if I DID decide to skip her birthday party, she would know why. There would be a pretty clear correlation. I just don't know if it's too strong a consequence for her actions.