I recently broke up with my ex a few weeks ago and tonight I was speaking with his sisters ex boyfriend as me and him have been friends for about a year but met eachother through our exs. We have been flirting quite abit and I really want to hook up with him as I'm quite sexually frustrated but part of me also thinks its wrong in a way? I know she still likes him but they aren't together so I dont see why I should really care either? Just don't know whether I should do what I want or try to care about how she would feel

If something in you like your is telling you that's wrong then you probably shouldn't do it. Aside from what I may think about the situation if your gut is telling you something or your conscience then it's always best to listen to what it's telling you.

Since you care you shouldn't. An ex wouldn't care about their feeling like you. So for your friend and sister sake don't do it. Just be honest to her how you feel about him and it also depends on the relationship you'll have with them too. It's best you tell her before dating her ex. You can tell her ex your feelings that's fine but let her talk to them about it so that way trouble out your business. It's worst them finding out and both persons unaware of your true feelings. You have all right to express it.

May your heart be your guiding key.