Honestly I know I should not care but there is a reason this is bothering me.
First I think you need to know a little context.
My gf after she moved to city for her higher education stayed at her sister's house. At that time I had not met her and she was talking to someone else. Her sister was dating someone else too. For some reason (which I assumed was just fraternal instinct) she was also in touch with the guy my gf was talking to at that time.
Things things did not work out between my now gf and her ex particularly because it was long distance and she hadn't ever met him irl and she started dating me. During this time her sister broke up and went all haywire. She went desperate in searching for a guy and thew all her tantrums at my gf. At one point she would hit her physically and my gf finally had to move out.
My gf got to talking terms with her sister again soon and we spend time together along with other friends too. Things turned messy when my gf's ex (who right now is just a friend) decided to drop in to say hi to my gf (and little did I know her sister). Before we knew it the guy was out on a trip with the sister, spending more time with her and voila they are dating.
I don't know why but I find this a really messy situation which I am not being able to process.
First, there is a feeling that the sister just used my gf to get to her ex which I find so convoluted and unethical. Considering how she had hurt her in the past I am suspicious she doesn't even care how my gf would feel about any of this. She seems to deliberately do things that gets my gf into a pickle and I hate her for that. Also she is way to old and it seems so immature and stupid for her to be involved with her little sister's past.
Second, I don't want this guy to be around this much considering a) he had already lied about his motive to come to visit my gf (which I think was just an excuse to meet her sister) and b) he is her ex and considering there is no way I can rid myself of this sister this would mean having that guy in shared spaces and gatherings in an even more weird situation.
I don't know if I am overthinking or if this valid. I just know I feel really icky and disgusted that something like this is happening and I am just in the crossfire of confusions. It'd be great if someone could help me process my feelings.
TL;DR: My gf's ex has started dating her sister when he had come to visit my gf as a friend. For context, sister had been abusive to my gf before and just keeps her like a means to an end, being absolutely indifferent to what she feels and dating her bf seems to be just another example of that. My gf feels awkward about all of this and I am in-between the crossfire of emotions, feeling disgusted by it all but feeling helpless since both of these people are important to my gf. Need help with processing what is going and what I can do.