Awkward & uncomfortable.

Was literally how I felt tonight. I went to visit my boyfriend who is currently in rehab and we had a pretty good time. Then afterwards his parents own a organic store and that also serves food and his mother invites me over there for birthday cake and ice cream being it's my 20th birthday tomorrow. Anyways, we have cake, I open my gift, and all that good stuff. My daughter wakes up and is hungry so I go into their office which has a big window looking into the store and I feed her. I left the lights off and the door open but I wish I would have just closed it 🙄. My sister in laws boyfriend goes past the door to go to the bathroom and when he comes out HE LITERALLY STOPS AT THE DOOR AND IS JUST STARING AT ME FEED MY BABY. I was so creeped out I couldn't even find words so I just turned towards the window and try to see if anyone else saw him staring. Then he proceeded to come back in the door way after he walked off after I caught him and said "i just wanted to let you know I used to be a womanizer but I'm not that guy anymore" like wtf. Way to dig yourself a hole. Any advice on what to do? I'm going to visit my boyfriend again tomorrow and I'm no doubt going to tell him what happened but how should I tell his sister???