Why does my boyfriends mother suddenly hate me?

My boyfriend and I are both college students, and have known each other since we were babies, even living together, so we are like family but we just started dating last year. His mother always liked me, and when we started dating she and I became closer, going shopping or out to eat together. Well a couple of weeks ago my boyfriend told her that he plans on moving in with me next semester, and that he wants to marry me after college. She started screaming and sobbing and she began telling him lies about me, like that she "heard" things about me and other guys. (Not true) and she told him that she would call the police because our relationship started when he was a minor and I was 18. He threatened her and almost hit her after that, and she told him that if he cared about HER then he'd drop me. WHAT THE HELL???? What have I done? The only thing I can think of is, she sees me as a threat and that I'm taking her "little boy" away. I am trying to be understanding of the fact that she doesn't want to let go of her first child, but there are reasons why I can't. First of all, his father died when he was 16, and even though his mom and dad were separated, for some reason since then his mother slowly started to pay less attention to her kids. She works nights 3 days a week and instead of being home the rest of the week, she goes out partying or stays the night at her boyfriends. My boyfriend is older and now out of the house, but his little sister is not and I worry about her. His mother is the type to "throw money" at her kids and leave them to fend for themselves. She will put a pair of $300 Dolce and Gabana sungles before taking her kids to the dentist. She is very materialistic and I remember when my boyfriend still lived at home, she spent $1000 of money that HE was supposed to receive in social security from his dads death, she went shopping and bought her, her daughter, and her boyfriend things but bought her son(my bf) NOTHING. Even before we started dating she would leave it up to me to take him to work, and pick him up, to feed him, to help with homework, etc. Which I was fine with, because I cared about him, in a best friend, family, and romantic way. The woman is trying to keep me from her house and her daughter, but her daughter misses me and asks me to come over when her mom is gone. She has even blocked my number from his little sisters phone. So him and I got on a family share plan and we blocked his moms number from both our phones. His entire family loves me, and they think she is being ridiculous. Why is she acting like I'm stealing him from her when long again she let me care for him? I feel like I was used. Help?!