WIBTA if I gave my mom somewhat of an ultimatum?

So over the years my little sister (14f) has had a habit of whenever she doesn’t like something I say even if it’s not directed towards her, she tells me to off myself, fully knowing I’ve tried and was in a mental hospital for it a few years ago. Both of my parents constantly let her insult me, tell me to off myself and let her call me horrendous names. My parents always excuse her behavior and never yell at her but when I stick up for myself I get yelled at and told I always start issues when she will come downstairs and harass me until I yell at her to go away, she has a problem if I look in her direction or breathe near her, like she still literally yell at me for these things and my parents will let her considering she has no problem doing it in front of them, and when they rarely discipline her she gets out of it by crying or throwing in my parents face that our half sister (18f) abused me and it’s all my fault (our older half sister has abused me horribly and has abused any animal I’ve ever gotten, and when I grew out of letting her abuse me she started on my little sister, one time she bashed her head against the toilet seat when they were in the bathroom together, gave her a concussion, and almost made her go blind because little sister took most of the hit near her eye socket. My little sister and half sister excluded me a lot because I couldn’t get over the fact she abused me and has called me the n-word (we’re all white and Native American)

So today we had to get out of our house for the day with our dog and my moms (baby) chickens and my dad set up a weird tent thing for us because we can’t really leave especially because it is 30 degrees and the chicks need care and heat, because a company had to do insulation work on our downstairs walls, and my little sister kept kicking the heater we have in there and knocking a long weird chick heater onto the baby chickens purposely by banging on the bin so it would fall and my moms chickens was outside the tent and asked what it was so I told her that little sister was being stupid and trying to hurt the baby chicks and in response little sister told me to off myself (she’s concerningly comfortable with saying this to others as well and will use it as an insult against her friends) and I told both my parents (my dad was at work so I texted him) and my Moms response was just to tell her to never say it again (which she has told her multiple times but there is no real consequence besides being told not to) and I’ll put my dads responses below.

Me- “R just told me to k!ll myself” Dad- “Ok well did you call her stupid?” Me- “No I said she was being stupid because she was banging the heater into the baby chickens which the heater has been falling on them all day because of it but it's crazy how that's justified to you.

But your wife doesn't care so I don't know why you would my bad” Dad- “What are you talking about i told R to leave you alone and she told me you called her stupid, thats all I know and you think im ok with it. You both need to grow up moms probably stressing and you 2 are fighting always”

So would I be the ah if I told my mom the next time little sister tells me to off myself that I’m going to do it?

(I would actually never do it but I just want to get the point across on how much this is effecting me and how much it’s effecting my life because it is a constant thing)