I need advice on Child custody / visits ( England )

Hi. Sorry if this is long. This is my last resort and really need any advice. So almost 2 years ago my father passed away. He had a partner I will refer to as “GF” and two kids ( my younger brothers ). So pretty much the kids parents was myself and my dad because their mother ( GF ) would never parent them in the slightest. So i took over most of her role as a mother to the kids because she would be sitting on the couch watching programs and ordering things online- Since my dad passed, she basically has cut us all off. She’s refusing to let me see my siblings. It’s been just over a year since I last saw them and honestly I want any way to know that they are ok. GF pretty much forced myself and the rest of my family to stay away from her and the children. No contact whatsoever. We tried but she either ignored us or threatened police action ( which would’ve been pointless ) we looked at any rights I had to see them even court ordered visits at that point. We got to the conclusion that unless she also signed some sort of agreement to let me see my younger brothers, no one could do anything about it. Did she sign it? No she completely ignored it. During this time, as I mentioned before, she never parented them, not a single bit. Always gave them little food that they also hated, never in-forced any boundaries or good behavior, and also continued to give one of them these weird ointments?? He’s already had a bad enough reaction to them to the point he was in hospital. But she never stopped giving them to him, and leaving them to play in the streets by themselves and never checking on them, sometimes leaving them until like 10-11pm. Pretty much, she was never an actual parent. The only people that treated my siblings well and actually tried to parent them was myself and my dad. To get to the point of why I’m writing this, I have a really awful feeling that something’s wrong with them but I have no idea what I can do to even check on them. I haven’t seen them for over a year and apart from the fact that they were my whole world, I know that she’s not doing what she should be as their mother. I need advice, anything. Any loophole that I can force her to give us visits. I need to see them. To know that they’re ok. To know that she’s actually at least trying to parent them. I will take anything. And obviously the extremes, the most I would want if I could dream, is to have full custody of them. I am yet to turn 18, but I will be soon. So if we did, it would be my mother that would be their legal guardian until I turn of age. Do I ever think this will happen? Absolutely not. But right now all I need to know is that they are ok, being treated right, and at this point are still alive ( I know that sounds extreme but that’s how much she never parented them ) If anyone could give any advice, if you know anything that I can do to order visits. I would really appreciate it. Thank you

( as a little extra, one of the kids also is very autistic. Struggles in school a lot, behavior issues, etc. Meaning he’s always needed extra help beyond just basic parenting, which also makes me worried. The other kid, also looks like myself and my dad. It’s like her genes never even touched him. And for some insane reason, she’s always hated him, her own child, just because he looks like me and my dad. The older kid, the autistic child, looks exactly like her, and she’s always favored him. It’s genuinely like he could never do any single thing wrong. I know that’s insane to say. It’s crazy to me too how she could treat them so vastly differently because of who they look like. But all of this adds to my other worries. ) Thank you for anyone who is able to offer advice. Also we have already tried things like CPS, they did absolutely nothing. Literally went to visit them ONCE and said nothings wrong? Even though the house was not at all a place where two kids should be living. Also she always gets them pets, but they are never treated right. Small cages, left in there uncleaned, never fed right, and because of how she ‘parents’ the kids, they think these delicate animals are plastic toys. ( rabbits, birds, and guinea pigs she has got them ) I don’t know if they still have them or any pets but that was what they had the last I saw them. I also only have a vague idea of where they are now living as they moved from my dad’s old house. Once again, thank you for any advice.