Advice please? (Sorry for the long post)

So I honestly really need some advice from people who don’t know me or my partner. My partner and I have been fogether for 5 years. We just had a little girl she is almost 4 months old. He has always been completely faithful until a little incident that happened last year. He and his boss decided to get really drunk and smoke a bunch of weed (no he does not smoke anymore now that we have the baby) and I was sending him some saucy pictures like I usually did. And for some reason I had a bad feeling so I went on his Facebook messenger and there was a conversation Between his brother’s wife and him about his niece and he was asking her how his niece was doing because they just had her and all the sudden she had sent my boyfriend a photo of her fingering herself and was telling him she wanted him, blah blah blah. He then responded “yum” and right after he said it he then sent “this is wrong please stop”. I’ve had a lot of trust issues with him but I’m starting to finally forgive him. (Yes I know a lot of people consider this not cheating but I do) but I’m just not comfortable being around her and I definitely don’t want her around my child (not only because she sent my boyfriend pictures but because she also has a problem with drugs).

I honestly just don’t know what to do. His whole

family keeps telling me I need to get over it and allow her to be around us and my daughter and I honestly just don’t feel like I can do it. 😞

They all say I’m just reacting and feeling this way for attention 🙄 her legit response to it when I confronted her was “I thought it was my husband” 🤦🏼‍♀️ how do you mistake your husbands name for his brothers