Care home sent her back

I used to work in a job facilitating discharges. The key difference between discharging to a care home and nursing home was unpredictable needs - care home staff can care for patients where it’s routine and they can give regular medication. Where a patient may vary, a nursing home would be best because a nurse can give additional meds where needed. My grandma was in a nursing home on the dementia unit and it had a sunny lounge and good care! You may need to look around but you can find somewhere.
Re your Step father - I would tel SS that you and your sister have POA and insist they contact you as well as him.
Good luck getting this sorted, I know it’s tough 💐

Oh, and no sedation. Doctors can prescribe medication if people are anxious/ distressed/ agitated but the purpose of that is not to sedate and if it has that effect they’d reduce the dose.

There absolutely won’t not be locked rooms or restraints!
There will be lounges, communal spaces, outdoor space (large gardens in some), activities, bbqs in summer, parties on occasion, animal visits, games, trips out, art , music, reminiscence, sensory stimulation, experienced staff, higher staff ratios than in residential homes… Go and look around a few, you will be surprised.

That's an incorrect assumption, dementia nursing homes aren't like that at all and you can't just sedate someone because they're aggressive, though sometimes the GP does prescribe medication to calm someone if it's going to benefit them. The homes still have communal areas and activities running etc. maybe go and see some and see what you think

This sounds so hard. She really must be cared for either in a home or by carers. If they can pay privately for care, do that - find a care agency or a home and just organise it irrespective of what SF wants. Sorry that that sounds harsh but once older people get to this stage, nothing gets done if you don't do it yourself!
If they can't afford it, you need to start being difficult! The people that you've spoken to have done what they all do as a first step, which is take advantage of helpfulness! You need to find out who the 'someone' is who called. They're presumably from some branch of social care?? (it's different for different local authorities but if you can't find any info online ask GP)
You need to phone them and demand (cry, beg, exaggerate, tell them the worst things that have happened) help.
The alternative (which may be what happens anyway) is that you wait for the next crisis, get her taken into hospital and then make sure discharge home is refused. Then something has to be sorted.
Best of luck with it all. It's a horrible horrible situation.

Do you have an assigned social worker? If not, you need one. Speak to the GP. The SW is the first advocate for getting her in the right place.

Sooner or later something will happen that means police or medical teams tell your SF that she is no longer safe to live independently. At that point your SW will identify a suitable home.

If she was violently lashing out at the staff in the home, she'll do that to a member of the public, and that will be the trigger.

You have my deepest sympathies, dealing with someone who, while it probably (hopefully) comes from a place of love, is making very bad choices on your mother's behalf.

You need a social worker to advocate for better choices.

How will the care home be funded? This will have an influence on how much the medical teams push for her to be in a home. (And might be affecting your SF's decisions)

I’ve work in a dementia unit and it’s nothing like you think. I remember putting on loud music and them all dancing and walking round the garden repeatedly with one of them looking at birds. It wasn’t perfect and occasionally we’d get hit by them but there was caring and kindness.

It just sounds like unfortunately you picked the wrong home for her needs. If it's not set up for someone then the home can't reasonably take in a resident who has more complex needs than they can cope with.
Can you just look at other options?

I worked in a dementia nursing home when I was younger.
No one was restrained or locked in rooms. They had a lounge and other communal areas. All their rooms had personal touches

OP thats very very unusual for a care home to not give someone a chance to settle in and to ring you and tell you to come and collect her. Was it actually you that spoke to them? Did you offer to go and get her or did they categorically tell you to?

You should have just left her to settle in. She can't leave without you taking her so she doesn't have a choice. If that happens again, don't take her.

She will be worse while she settles. The move will obviously be frightening for her, she will lash out, but the home should expect that and have experience.
I know two people in dementia homes, they aren’t locked in, they go into the lounge.

Have you had a social worker in to assess?
Was it a home specialising in dementia, or just a care home?