My Mom 48F is dating someone close to my age 24F. Any advice?

My mom 48F is dating someone close to my age 24F. He is a 28M. To make a long story short they are coworkers which makes this all worse than it already is. I want her to be happy but I just can’t accept this. Especially since I have a little brother who just started middle school living with her who she is now leaving at home by himself or he is living with friends while she hangs out with this guy. This makes me want to go no contact with her. I don’t know what to say to her to express my concerns and disgust respectfully. Does anyone have any advice? I want her to be happy but I’m weary of this guys intentions and her just leaving my brother to fend for himself. She mentioned this relationship being a potential months ago and I told it’s not a good idea but she’s now telling my brother not to tell me about it and keep it secret he’s told me but she’s not aware I know about it