If you have brought up these to your pediatrician and they haven't been worried, I wouldn't worry about it.
My son is quite tall for his age and always had a harder time sitting up, he didn't exactly topple over, but he's long in the torso and his center of balance is off because of this (his is up top, where most kids are centered). If you can afford it, I'd try purchasing a bumbo. My daycare provider had one of those and sat my son in it for a bit each day. He learned to keep his balance, it also helped I think with getting his sides strong enough to help balance him, but still had a little freedom to move.
My son only army crawled until about 8 or 9 months, then he started the real crawling. He's no 13 months old and cruises along furniture, but his high center of gravity makes him wobbly so he's afraid to let go and take more than 1 step on his own.
Bottom line, if you're worried, I'd just bring it up to her pediatrician and if they are not worried I wouldn't be.