@Shaz51
Maybe eye ball your mum about your real concern for her welfare, and listen to her response. Make a decision whether or not its right to go over your mother's stated position. The sister's opinion probably has less weight, unless she has been granted medical attorney ...
Listen to your own instinct if your mum is she being casual, but covering up
It is the hard fact that we cannot see inside another person except with wonderful modern and expensive medical imaging, and even that takes interpretation....
I am going through a lot of these doubts with my son, but as it is a different relationship and our life stage is about me letting go and him standing on his own feet, it is better to err on the side ... of letting him "have a go" .... whereas with your mother maybe caution ... is important. Tbh My mother would NEVER have allowed me to make decisions for her. She was very proud. FOr a while I was the driver to all her medical appointments, but it was always set up with me after she made her own appointments. She was in Palliative care for 9 months so her call was very accurate about her own ability to cope and her own health.
I have an old friend in a dementia ward and it was traumatic for me to visit him. I went last year. It was all very fine and he was holding my arm, but my realisation of his mental decline took a toll. I thought I should visit every week, but have had my hands full, just with my son, my cat, garden house etc. He has remained stable but very non functional according to his wife. That is one I have stepped back from. I have another friend in another home ... for a while the only thing my son and I did was visit those homes .... weary sigh ... cant say whether people are better off in or out of them ... it all depends ....
Usually if a resident has family visiting they are at less risk.