Some messy tea

So I got myself involved in some terrible drama by hooking up with my ex boyfriends best friend. I feel so guilty about it but at the same time I’m oddly happy with the current fwb situation. It’s just fun and no real commitment is needed. We both talked about and know that nobody can find out about what we’re doing because since we’re in the same friend group it would create chaos and ruin friendships. I can’t talk to anyone about the current situation because my go to person outside of my friends is my sister, and if my sister found out she would most likely not have the best reaction (to put it lightly). I know that people’s reaction to this situation will be mixed at best but I really need advice on how do work with this situation. I know what I’ve done can’t be reversed so should I just keep rolling with the situation and follow the fwb through even though it’s a pretty shitty thing to do, should I stop doing the shitty thing even though I’m quite happy or should we confess and tell people what we did and come clean?