Throwaway so my family doesn't connect this to my main.
I'm a 23 year old childfree woman (idk if this is needed for the story), and my sister is 30 with a 4 year old son. She's a single mom, ex-husband is not in the picture and has no contact with her or the kid. I'm gonna get straight to the point, that kid is a nightmare to be around. It's not his fault, it's my sister's fault. He very clearly has behavioral and mental health disorders, even at his young age. However, my sister was and is very ablest and refuses to accept that her son has problems. The kid screams bloody murder whenever something doesn't slightly go his way, screams and cries when he's overwhelmed, has meltdowns to certain sounds and textures. Like I said, I don't blame the kid for these issues, but my sister refusing to get him treatment or help makes the kid a nightmare to be around. So, as a result I don't babysit the kid when my sister asks (more like demands) I do so. This causes a lot of fights between us and our family.
I have a friend, 23 M, who's married to a wonderful lady age 22 and they have a baby boy together who just turned 1. Yes they got married and had him young, but it's not my place to judge them. This past weekend they asked me to babysit for them, and I agreed for a few reasons. Their kid is calm, well behaved, and a general good kid/toddler/baby to be around. I love babysitting him. Also, they asked me nicely and understood that I might say no because I'm childfree. They were polite about their request. A thing about me is that if you ask me to do something politely, even if it's something I wouldn't usually agree to, I'd probably do it because you were nice and polite. So for these reasons, I happily agreed to babysit the boy. My sister also asked me to babysit her son that day so she could have a day to herself, and I refused and told her I was already babysitting for my friends. She was pissed, to say the least. She said a few comments about how I was an ass and not a good sister, but I thought she dropped it after that.
This past weekend, while I was watching the baby, my sister started banging on my door. I looked through the window and saw that she had her kid and a care bag with her, and I knew she was trying to drop him off and make me babysit. She was banging on the door, yelling at me and calling me heartless and a baby hater (while I was holding a baby, ironic) and that I better open up and do my sisterly duties. I put the baby in a different room to keep him from getting scared by the noise, and told my sister through the window to leave, or I would call the cops. She started cursing for a solid five minutes before she finally left. I had the whole thing on my doorbell camera and sent it to the family group chat. Surprisingly, my whole family is on my sister's side.
She keeps arguing with me about how I'd rather babysit a kid not even related to me over my nephew. I reminded her that her kid is a nightmare to be around, but she keeps saying that for family you help out. My sister has been cold since, and keeps sending me text messages to provoke me into arguing with her more. She's also started saying some pretty ablest and nasty things towards my friend's wife. My friend's wife is on the spectrum and has BPD and my sister keeps saying that she should have been sterilized and never should have passed her issues to her kid (btw, her kid has shown none of the signs that autistic infants usually show so idk what my sister is on). I will admit that I do feel a bit guilty because I never consider babysitting my nephew, but I jumped at the opportunity to babysit my friend's kid. My sister, dad (66), mom (64) and brother (33) are all calling me an asshole and saying that I'm just using the CF label to avoid family responsibility. I need advice on how to get my family to see my perspective on this. How can I explain to them that the issue isn't the idea of babysitting, but it's the behavior of the kid?