Can't allow my DS to go to my Mum's house

[MommaLM]

I have had issues with my Mum my whole life. She is a very self-centred person and exaggerates everything whilst not listening to anything else anyone says and trying to bring it back to her.
My main issue at the moment is that my sister who is 18 still lives with my Mum. She is extremely aggressive is her speech and mannerisms even though there are always children about. I have always made it clear she isn't to act this way around my DS who is 20 months old.
Last week when we were there, either my DS or my nephew who is 2 years old turned off the bathroom light (midday, so plenty of light regardless) and my sister started shouting and swearing to have someone turn the light back on. I was shocked to walk down there and see the two toddlers were standing there with the bathroom door opened just watching her screaming and swearing. I got extremely upset with her and she would not apologise for the way she acted and it only got worse. I told my Mother that my son wouldn't be round again because of the way she acts in front of him and she has done nothing about it whatsoever.
My DS possibility has autism and is more sensitive to temperaments and noise and this seems to mean nothing to my Mum. I tried to talk to her about it again and she acted like my sister never acts this way and seems to not be upset with the fact her grandson won't be coming back round.
Am I right in thinking that she should be dealing with this issue?

I dont think you can control what happens in someone elses house, especially as your sister lives there.
Having said that, I;d be making damn sure my DC didnt go round there again! Children do not need to hear that sort of language or be shouted at like that. I'd also tell your nephews parents how their son was being treated.
Maybe if your mum doesnt seem bothered about your DS possibly being autistic, you could back off. Or you arrange to meet your mum in a cafe or somewhere so you can still see each other but hopefully without the horrid sister?

It is just frustrating as my Mum has not once tried to speak to my sister about this and is now just denying that she ever acts like this. I don't want to have to miss out on going round at Christmas with my DS but I would never allow that to continue.

Your mum probably thinks you’re going to give in and bring your child back. Stick to your guns, op. Your child doesn’t need to be around your arse of a sister.

I can't explain enough how it is with my Mum and sister. My sister does nothing for herself, her meals are made and brought to her room, she won't walk to college alone, my Dad has to walk her, she acts like a child.
My Mum could easily say to her that if she doesn't behave when my DS is around it means she can't see her Grandson but she just won't do it. I agree however that I can choose to just not go round etc.

Not dominate, just reason with her as it is her house, which my sister, pays nothing into.