Instead of texting her at 2 a.m., wait until morning. Text her after 9 a.m. It's pointless to wake her up in the middle of the night. There was nothing for her to do right then and there.

How would assisted living help? Wouldn't she still be alone at night? Or would staff there check on her every few hours at night?

If your sister is P.O A., then she pretty much calls the shots. You could challenge that before a judge, if you are willing to say that your sister has been irresponsible in handling your mom's affairs and has been neglectful. That would make you and your sister enemies for life. I'ld be slow to do that, if it were me.

Sometimes you just have to allow a situation to get worse before anything constructive is going to happen. Adult Protection Services is very slow to force elders into a more supervised living situation. (Been there, done that.) I know you are concerned for your mom's safety. But your options are limited.