Advice needed

I recently went on a weekend trip away with my brother, his partner and their two children. My brothers partner hasn’t ever been what we would call a ‘good’ mum. She is addicted to weed, smokes daily, spends around £200pm on it. When we went away, she didn’t bring any with her, i asked her why and she said due to the security around the holiday campus. She is a completely different person when she hasn’t got weed, she is vile and just horrible. She barely ate anything over the weekend as she was cold turkey from the weed. She spoke to my nephew like shit for the whole weekend and barely did anything with her youngest (4.5year old). The youngest is still in nappies (she hasn’t bothered with potty training) while we was away she would get him dressed and ask ‘do you want a nappy today or wear pants’ obviously he chose a nappy so that was that. She has hit the children before also. We went swimming, the youngest was having a meltdown after having to get out the pool and my other nephew was giving attitude apparently, she came out after swimming proudly saying ‘well they both got a slap off me’. I’ve spoke with my brother twice within the last year saying he really needs to leave her, the kids show no love to their mother and she is just vile. Now.. i am seriously considering calling social services regarding the children but unsure whether to give my brother an ultimatum first (leave her or i’ll call them) or just call them anonymously.. My brother is 100% being emotionally abused. After the weekend trip, i called him on his lunch break and had a big chat with him about life, he wants to leave her but knows his life would be hell if he did and he only stays for the children. He works full time but when he is off work, he plays the role of mum and dad, he does everything for the children, he’s the most patient, kind hearted man you could imagine. There is so much more to this that i simply don’t have enough space to type it all out. Mum has never took her child to a toddler group, soft play, no childcare setting. She works all hours under the sun (doesn’t have to as my brothers income is adequate) but she would rather work than be at home with the children. The youngest child gets taken to whatever family member can have him while she goes work. He has no routine, goes to bed when he wants. Still has a baby bottle with hot chocolate several times a day, just generally unkept. The older child believes there is something wrong with him mentally because that’s what she has told him. He is 13 for reference. My brother told me that she unbuckled her seat belt on the way home down the motorway to slap my 13 year old nephew and then kicked him out the car not long before arriving home as she had had enough. He spent the night at a friends house. Just want advice. Thank you