<human>: Can you provide a brief rundown of the text in within 3 sentences?

So my mother has been divorced from my father for a very long time, as long as I can remember. She has not seen anybody in the last 5 or so years because of her run-ins with abusive boyfriends. Recently however, she met a man that has made her pretty happy and he is a really decent guy. It's a wonderful thing in my opinion and it's great to have my mother happy again... here's the problem though.

That guy is the father of my brother's girlfriend.
My brother is having a really hard time with this, he believes that because of my mom getting with this guy that he can no longer be with his girlfriend, who he has been with for about 3 or so years, because it would be incest.

How can I explain to him that this would not be incest and that he shouldn't throw away this relationship with his long-time girlfriend because of it?
<bot>: Mom has a new boyfriend, it happens to be my brother's girlfriend's father. He is freaking out because he thinks it's incest, how can I explain it to him?