Am I being the arsehole?

So a little bit of background (sorry it’s a long one!!!) I have a sister who is 3 years younger than me (aged 30). We have never been close as we’ve always been worlds apart, she was kicked out of school for doing drugs, got pregnant at 16 and has since had 3 kids with 3 different men (no judgement on that, but it’s an important part). I went to uni, worked hard in my job, bought a house and had my daughter with another on the way. I was only 20 when my niece was born, and although I did not see eye to eye with my sister I ended up having my niece most weekends from Friday after work to Sunday and whenever I had time off from work/uni. Her boyfriend at the time left her and she went massively downhill with drugs and alcohol. I had my niece for about a week straight and went to drop her off back to my sister and the flat was full of men, so I immediately drove off with my niece and between my mother, and grandmother we cared for her whilst my sister went on a drug binge. She then fell pregnant again and didn’t know who the dad was, and her pregnancy was hell for the whole family. She’s always had an aersol sniffing addiction and this got worse during this pregnancy. We were crying out for help from all professionals and were honest about her addictions and nothing was done. Over the years there have been multiple social services referrals and nothing ever comes of them. I ended up having my nephew every weekend as my niece would go to her dad’s (he’s a lovely man luckily). Her second child is now basically living with my grandparents. 4 years ago she met someone new and seemed to turn over a new leaf. Our daughters were born within days of each other and we ended up spending more time together as our girls love eachother. 2 years ago that relationship broke down with her new partner and things have been on the steady decline. The kids teeth are rotten, she doesn’t send them to school when she does it’s with a mouldy packed lunch. All of this has been reported to social services by the school. She’s now taking cocain, my mother and grandmother have the kids as much as they can (it’s been harder for me to be involved since I now live a 30 minute drive away and have since started my own family and work full time). My sister had one decent friend who was on the straight and narrow, but she’s also had to cut ties as she’s just had a baby and does not want to be around someone like my sister and the drugs and the people she hangs around with. I’ve lost all patience with her, she has all the help in the world, yet doesn’t want it. The kids are going under the radar each time with social services. My mother is constantly guilt tripping me about not doing more for the kids, she even told me I should be buying them clothes as I buy my daughter enough. Meanwhile my sister is out getting lip and nose filler. Before I found out I was pregnant I was told by her I should take my niece on, and I could easily afford to pay the nursery fees for another child. When i responded with ‘what if I want another baby’, it was immediately ‘you won’t need anymore if you take on your sisters kids’. I’m on the verge of not wanting a thing to do with my sister, I will see the kids through my family as they have them 60% of the time. But I cannot associate myself with someone who is a drug addict and neglects her children. She didn’t even turn up to any of their Christmas concerts. Since having my own children I struggle with this even more! My mother thinks I’m being unreasonable and it’s my pregnancy hormones, but I hate her for how she treats the kids!!