OP I'm so sorry to hear you've had to go through this. We had something similiar happen from SiL after a stroppy comment from me at a family get together. It wasnt photos but complaints about our house, saying I was suicidal & we were not looking after our dc correctly -neglect & not feeding them. SS turned up when I was about to take my dc to swimming lesson. They made an appointment to come back & I had a friend who knew me well & hubby there when they came. SS looked at our house, our fridge. Spoke to us & the children (together & separstely) & they said it was obviously an unfounded malicious complaint & they would not be taking it any further. I had no idea who had made the complaint (SS keep that confidential), just that from things SS said it was someone who had been in our house.
I was absolutely devestated and so was my husband. I had friends and kids from my sons' primary school & preschool over our house all the time. I was completely paranoid & upset that someone locally I knew was saying all these things. Then I got the written report from SS about what the complainant said & what SS said about their visits & the conclusion it was a malicious complaint. I knew from what they said the complainant said that it had to be from a specific day when the family were at ours. The only people were me, DH, our kids, my DM & DH's DPs (who would not have done that) & SiL & BiL & their DCs. I was bringing my DCs up to write or draw Thank Yous when they were given gifts. My BiL said not to bother, I said our DC were being brought up to say Thank You. That was it. But there was an awkward annoyed pause before things went back on track. Anyway I could tell from the SS report it had to have been SiL (& I then thought BiL).
DH & I talked about it. He wanted to have it out with them but I said no. Mainly because his Sister was his DPs def favourite child & they did everything they could for her & her dcs so I thought it bringing his DPs into it would mean they would side with her & damage his relationship with his parents.
I blanked SiL. They didnt see our kids & his DPs saw them when SiL would not be there. His DPs werent that bothered as our DCs were the wrong grandchildren. They never had our DC over without us or babysat before or after the incident - well maybe babysat 2-3 times as our DCs grew up. My PiL were always quite transactional - visits on their terms when my DH could do something for them. They had a history of being uninterested or unwilling to help with our kids even there were emergencies like my son being so ill with chickenpox I needed to take him to hospital to be diagnosed, or when I was hospitalised with suspected menengitis when DH abroad with work. Hence I felt DH would be the loser in any argument between him & his sister. Which 6/7 years later I was proved right about when his sister was getting a divorce & admitted what she had done. She never ever said sorry.
Decades later I still get upset about it. Angry & upset. OP its really horrible to have that happen. I know how painful it is. I think you are right to block your sister from having anything to do with your DCs. As a parent you have to protect your children which is exactly what I did. If your mum wont accept that then you have to decide if you will only allow her contact with you there. Tnh thats what I would do, & if thats not acceptable to her Id block her.
I would point out to her that not only are you very upset, and that your sister put your DC at risk at being taken away, but that she has put you and DC on notice and flagged up. I say this because SS went & talked to my DCs primary school. I went & spoke go the Head & class teachers sobbing because I felt they needed to know. They already knew. DS services had talked to them. That stayed with their records throughout school. Even at Secondary when my youngest went in with a singed eyebrow from a Scouts outdoor cooking session the school wanted me to explain why & how it had happened.
It doesnt flag up on any DBS checks even Enhanced. Ive had multiple DBD checks over the years because of working with children, teenagers, vulnerable & mentally ill adults & nothing. After all you & I have done nothing wrong.
However with one application the organisation asked separately if I had ever had SS contact me re this. I made mistake of answering truthfully explaining circumstances & was grilled to supply evidence, SS report & details & had an indepth Zoom interview which I have to say brought everything back up again & was really upsetting. I know the organisation were only being really carefull & I still got the position, but the process was just really deveststing all over again.
So be aware to keep your SS report exconerating you in a safe place. At some point it may come up.
So no your sister & your mum if she supports her deserve NC. Your sister has done lasting damage to you & your DC with this.