Is it legal for a person with Dementia to buy a house?

Hi, I don't know if anyone here can answer this question, but I'm freaking out a bit (I have an anxiety disorder, by the way). My brother, myself and my mom just bought a house. My mom has Dementia and we all live together, and my bro and I take care of Mom. Anyway, the thing is, my mom signed all the papers to get the house and it's in her name (otherwise we would have had to pay gift taxes if it was in my brother's and my name, too). I'm freaking out 'cause I'm wondering if it's legal for a person with Dementia to do all that. To buy a home. I don't even know that she understood what she was signing. So, I think - was she actually lying (not purposely, of course) in reality if she didn't understand what she signed? My brother took care of most of the business of buying the house, but it was my mom's money that bought it, and she signed all those papers. Is it legal??