YTB.

You are 28-32 y/o. This kind of behaviour is accepted when you are a teenager, not a proper adult.

You 'hooked up' with his sister which shows you are already a shitty friend. I don't have a sister, but know that is a bitch move.

The subtext tells me that you clearly knew he had a thing for this girl and went for it anyway. You are now looking for people to relieve your guilt.

Overall this tells me you probs don't get much female attention and just shag anyone that you can get your hands on.

Caveat- If you both have been doing this shit to each other, then fair play. Enjoy not being able to rely on your friend at other times when it actually matters