Though this may be a short duration problem with your mom the age she is, I would strongly encourage you to see that attorney and get this stuff established more specifically very quickly. I will tell you what will likely happen with your mom. She won't want to fight or disagree with either one of you. When she's being talked to, driven around, taken places by sister and husband, she'll agree to whatever they want her to agree to, up to and including signing papers that give them complete control (and believe me, I have seen this happen!)
And then when you start to question her, she 'won't remember' because she doesn't want to get into a confrontation with either of you. In other words, she is in a prime place and very vulnerable to being bullied and coerced.
Do not be surprised, if and when you ask her to come with you and talk to an attorney, even though you tell your sister (and unfortunately, she will want to include bully boy in this too if you don't insist he not be involved) about such a meeting and ask her to be present, you are then accused of trying to do this very thing. Your sister will probably tell your mom that YOU are trying to get your hands on everything and exclude her. And your mom will retreat into "I don't want to make either of you mad at me. I remember nothing! I don't want to change anything!"
But unless you are willing to step in now, before your mother declines any more, do not be surprised if she at some point is not talked into what they have already tried, in other words, "We're going to take charge of your situation, mom, ammo (you) doesn't want to be involved!"
