what is wrong with my niece, and how can i help her.?

my niece turned 1 year old on oct1.
she is a happy, giggly, smiley baby. she wakes up happy, goes to sleep happy, and generally doesnt fuss very much at all, and hasnt since the day she was born.
but the problem is, she is over a yr now and does not even sit up unassisted, she can sit for 30 sec to a min and not fall over, but then she just loses her balance and falls over.
she does not crawl, doesnt 'furniture surf', and is yet to make any clear words, or forms of words.
when she gets really excited she will throw herself back really hard(ive gotten hit in the jaw a few times, and she has some power to her).
her legs are strong, when she is in her walker she gets around, mostly backing up though, and she loves jumping in her johny jumper. she can roll over no problem, thats how she gets herself around for the most part lol. pregnancy question and answersNewborn&Baby
i would say maybe she is just a little behind and will catch up, but she cant even sit up and is a yr old already....
she is in the custody of my older sister currently, and her mother gets out of jail in a week, im really worried because her mother (oldest sister) wont take the time to work with her, she uses my niece and nephew as objects to get what she wants from people, she passes them around to who ever will take them, especially the baby because she didnt want her in the first place, and it was even worse to her that my niece turned out to be a girl instead of the boy she wanted.
anyways what could be wrong with my niece?
my sister smoked weed through out her entire pregnancy, so i assume that has a lot to do with my nieces delays.....

this is something that her pediatrician should have picked up on by now, if not you need a new one and someone in the family needs to sue for custody so that the abuse from the Mom doesn't continue

wow, i'd tell a doctor or social worker