Aibu to tell my family my sister is in an abusive relationship

[Doubtfuldaphne]

My sister tells me everything her dp does. It's all bad. He's absolutely awful. I feel it's my job to talk to the family about it. She knows I do. She has started talking more openly about it with everyone too.
But, when I found out through my mum that she had recently done something incredibly stupid for him I confronted her.
Last night I got a call from my mum. She was so upset saying I'd gone behind her back talking to my sister and she doesn't want my sister to think we are all talking behind her back.
We are just bloody concerned. But no one wants to upset my sister!
My dad who is separated from my mum is starting to really like him which is just he worst scenario possible.
Im wondering if it's just easier to never talk about their relationship again just for a bit of peace. As soon as my sister gets over the latest ordeal she becomes so defensive of him again!

OP ,, Until your DS opens her eyes, see's what he is ... everything will fall on deaf ears I am afraid . She either won't admit it or will go on the defensive, which seems confusing to those not living it.

I lived it . My DPs, DS, DGM, Auntie, all tried to get me to kick him out. I did eventually, when I was ready and strong enough emotionally,, He has been gone 5 years now.

yanbu. Good luck with that mess.

Talk all you want. It's not going to change your sisters view of this relationship.

Be there for her, listen & support.

That is all you can do.

Keep talking and tell your mum this is the healthy way, not his way of controlling and distancing.
Your sister will wake up and knows that you love her and tell your father all about too
Honesty is the best policy with abusive arseholes, the people know about him the better

sweeping it under the carpet is what keeps evil alive.
tell DM and DF all you know.
your sister needs help and real support.
silence will allow the abuse to continue.

If she told you these things in confidence then no, it's not your place to tell everyone and you may ostracise her and she may feel she can't confide in you in the future. I'm wondering why she told your mum about the stupid thing and hid it from you. She will be having complex emotions that you don't have and if she wants everything in the open then its her choice, not yours. If you back her into a corner through your good intentions then she may feel as though she has no support and is less likely to leave.
Ultimately the choice has to be hers and you have a limited influence as to how and when she leaves.

Give her the number for Women's Aid, tell her that you will be there when she leaves and support her. Its hard but only she can make that decision.

You are all worried about her and care about her but a word of caution-talk to her directly and not to other people. Your intentions are good but she may not see it that way and feel that you are betraying her confidences.(I have learned this at my cost).
If she feels everyone is talking about her she could stop telling you anything and become more isolated. She may even fall out with you.