That's right, you did mention a jaw disease in your O/P.
That's so sad for your girl.
Yes, brushing her teeth at least once weekly would be good for her.
I have never had to brush any of my babies teeth so I can't offer much as to what toothpastes would be good or bad but, do let us know what you decide to use and how it works for her teeth as well as her system in general.

P.S.
Don't forget about her gums and tongue as well.
The gums and tongue can harbor a lot of bacteria that can aggravate any existing condition and of course contribute to many.

I use just a soft bristled tooth brush or a "tongue pad" for my babies tongues to gently remove plaque and gunk and I, if needed, will take a damp cloth and rub their gums with it as well as the roof of their mouths and under their tongues.