Not sure about my sister as she is older and I don't remember, but my mum certainly brushed my brother's and mine until we were about 10! Not because we were incapable in any way (sleepovers at other kids' houses weren't a problem or anything), but merely because she was always going to do a better job until we were old enough to really care about hygiene, cleanliness and keeping our teeth in good shape.
I'll be 30 in 3 days and I've never had a filling or any sort of work done on my teeth, other than getting braces in my 20s purely because one tooth was slightly out of line and I was incredibly vain. My brother is 3 years younger and is the same.
Brush them as long as she will let you!