It is bad I hook up with my ex wife?

We divorced very amicably, we kept in contact as friends. I had other relationships and she did too but last month we started being closer and getting intimacy again and I believe she's way better than the other women I frequented after our divorce... she said she feels the same and was disappointed to find how many losers are around...

I am not sure what's happening here but for now we are good FwBs. Problem my sister dropped in at my place and saw both me and my ex after I opened the door. She was furious and said this is no good and in her opinion it should be illegal to hook up after divorce...

Am I making a mistake here or what, my sister babbled with my parents and they are happy bit cautious about this...

It is bad I hook up with my ex wife?
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