Am I over reacting?

So my boyfriend of 2 years has a sister that I dislike ...extremely dislike. I’ll just start off by saying I didn’t always not like her. We were okay at first, but then I started noticing how crappy of a person she is.

So storytime: I started living with his family (parents and the sister) after about a year of being together. Things were okay until one day she got upset about her towel being damp when she went to use it. She assumed that I used it and texted her brother charging him the rent for the month, with an additional $25 added because he has a “guest” living with them. The “guest” is me. That was petty and something I could have taken VERY personal, but took it as a misunderstanding and let it go. My boyfriend got upset about it and talked to her about it with me in front of her and she had nothing to say because she was so embarrassed.

After that incident I just started seeing her talk crap about people behind their backs. She always liked to talk stuff every chance she got. Nothing she ever said made her seem like a nice person and I just didn’t like that. So then something bigger happened that made me just not talk to her at all.

Storytime: At this time she’s now planning on moving out (but still in the same apartments we live in). She loaned my boyfriend $500 for his DACA renewal. She said he could pay her slowly back in payments. And since she was moving, he sold her computer for $250. He listed the computer online for $300 but was giving her a discount. Later on, the sister got HER bf to use the account to “joke around” and offer $200 on the online listing. The next day, the sister assumed they were now agreeing on $200. Wrong. I told my bf no, that’s unfair to you and it’s going to be sold to her what you first agreed on. So then a few days later she asked my bf to help her move furniture, my bf said no he’s busy and she got mad and said “I need that $500 back now and I’m going to consider giving the computer back to you”. She was being so petty at that point.

So in the end she tried to cheat and steal money from her own brother. Seeing how much that angered and hurt my bf made me just cut ties with her finally. I know it wasn’t personal to me, but if it affected him which affected me. Since then I haven’t spoke to her and don’t even want to bother with her even though I see her every other day. Am I being too hard?