I feel like my son and his girlfriend are stealing my daughter from me.

My son (17) and his girlfriend (18) spend a lot of time with his 11 year old autistic sister because I'm at work a lot. My son takes her everywhere he goes with his girlfriend which I don't approve of because I know that he's having sex with her even though he lies and tells me he's still a Virgin. He always buys her new clothes and they buy gifts together for her. I've seen my son and his girlfriend sitting in the living room with his sister teaching her how to focus on listening more instead of constantly talking. My daughter recently had her first period and neither she or my son called me. He bought pads and tampons then called his girlfriend over so she could show her how to use both. I started crying in the shower earlier because these were supposed to be moments with ME. I told my son yesterday he is not her father and that girl is not her mother & to keep their young asses in their place. He looked hurt by my statements and stormed off to his room. I realize I went off on him but I feel I was justified in doing so. I don't know how to feel about anything right now but I know it hurts when I hear my daughter talking about going to my son's girlfriend for all her problems and concerns over me. I know I'm always at work but I'm a single mother trying to make sure my children don't end up on the streets homeless. I just want to be close with my daughter.