In love with my boyfriend's step brother

I know you all are gonna call me horrible for this. I've fallen for my boyfriend's step brother. I've been with my boyfriend for 7 years. I've been really wanting to get married and have kids. I mean I'm 33... Clock is ticking. But he won't even MOVE IN WITH ME!!! He prefers living at home with his dad and step mom and step brother. None of them work except his step brother. His family is extremely mean to his step brother because he's autistic. He's extremely sweet though. He works in construction. His dad and step mom take every single penny he makes so they don't have to work and then bully him. They're all huge bullies. I get my boyfriend a gift every year for his birthday but he's never even given me a card but his step brother makes me something every year. Tuesday all I wanted for my birthday was to see a movie. Specifically Shang-Chi and the legend of the 10 rings. My boyfriend said he was taking me to the movies for my birthday... I just had to buy the tickets... I bought them and when it was time he didn't want to go anymore. He slapped his brother on the head and told him to go with me instead. He got excited and wore his avengers shirt. We had an amazing time. I had a talk with him about how I wished his step brother would get serious with me. He said he hopes he does because he wants me to be happy and likes seeing me happy. He gave me a rose for my birthday. I've been falling in love with him. I started getting even more defensive when his family abuses him and actually went off and told his step mom and dad to get jobs and they're lazy bums. My boyfriend called me an annoying bitch. Yesterday his brother wrote me a letter saying he knows he's not supposed to like me but he does but knows it's wrong. He just didn't want to lie to me and says he hope his step brother gets serious and I have everything I ever wanted... I'm in love with this man.

I honestly think his step brother needs to cut off his family. They treat him like absolute crap for no reason but he has no one else so he puts up with it and is too sweet to defend himself.