It's so hard to not compare your baby with others..most parents seem to do this.
However, you know your baby best and if something concerns you, I'd look into it and be persistent with it to your doctor. If time goes on and she's still not standing with assistance, after 6 or so months, check back again with your doctor and it never hurts to get another professional's opinion.
I think your baby is doing just fine, but you know her better than any of us that are responding to you do, so trust your instincts if you are ever concerned about anything.