As she gets older her hair will most likely fill in but some people just have thin hair . If she is eating a good diet and is healthy I wouldnt worry too much But also be careful with hair ties as they can pull out her hair, They make baby hair ties that have velcro on them just be careful she doesnt pull them out herself.
Does her mom and dad have thin hair also you said it was genetics?
Yes vits are good , if you're child is eating a good diet but you are still concerned about their hair, try trimming it regularly. Do not use blow dryers (at least this is what i heard) and stick to natural based shampoos and conditioners. Try not to tie hair back in a pony tail so often, as this can pull the roots and cause even more hair loss, thus leading to less hair.I have also heard vinegar is helpful but as she is 2 year old be careful what you do put on it ..I think it will grow thicker as he gets older.