My (F26) sister (F31) has befriended a fox and is letting him inside of her house

My sister lives on her own with her child (M4). We get on really well and I used to regularly visit although with the pandemic this is something that I hadn't done for a few months until last week.

I went over just to catch up and have some lunch with her and my nephew and whilst I was there there was a fox in the garden outside. Apparently this fox has been hanging around her garden for a couple of weeks now, she's named it Tony and I was horrified to hear that she'd let it inside the house and fed it some chicken.

She didn't seem to think letting the fox into the house with her four year old child was a problem and thinks that he's friendly. I pleaded with her not to let the fox into the house and that it was a wild animal and could hurt either her or her son but I really don't think she took any of it seriously. She told me that he comes around every day in the afternoon and that he's basically like a dog.

I've sent her links about how dangerous wild animals can be and advice that you should never try to interact with them, I've gotten other family members to talk to her about this (apparently our Mum knew about this and thought it wasn't a big deal either???). She still doesn't seem to think this is a big deal and that I am ruining the fun since always been really friendly and my nephew is pretty attached to him too now.

What else can I do to get through to her?? Am I just overreacting here? It's even more concerning to me because her son, my nephew, is so young too.

TL;DR My sister has befriended a wild fox and been letting him inside the house with her 4 year old son.