I think it is a mistake, and not for the reasons mentioned thus far, or at least not only for those reasons.
I don't think you are helping her by just throwing money at the problem, and making her reliant upon you. Instead, if she wants help, I would help her by finding what programs she qualifies for (WIC, disability, Section 8 or other housing assistance, food banks, etc.), helping her set a food budget (find a stable of cheap meal she can cook), helping her look for a cheaper place*, etc.
That's a real gift, not throwing your money at the problem and creating a situation almost guaranteed to degrade or destroy family relationships, while not showing her how to live within her means in the long run and also uprooting her from her job, support system, etc.
If her credit won't allow her to get a place and you are so willing to put your own finances on the line, consider cosigning for her. Not something I'd usually recommend, but if you are hellbent on going that far to help, to me that makes more sense than buying a house and moving her in. At least that way, after a year you can pretty easily (or at least more easily) walk away, than if she's a tenant in your rental. Or even prepay her rent and have her pay it back to you monthly. Again, you are at risk, but at least it's only for a year.