I didn't know what category to put this in…

So my best friends mom is dating this guy with a son the same age as us (15). My friend just told me that her and the son are now together as well. I tried talking to her about it because I feel it isn't a very good idea to be in a possibly sexual relationship with your potential step brother. When I tried talking to her about she was very rude to me and my friend about and saying that there was nothing weird about it. To top it off, when my friend and I followed him on social media she got very mad at us and told us that he didn't want us following him. I  don't know what to do and need advice because I have a feeling that it could end up being very toxic.