Depending on where you are, there may be some of the new region restrictions. (Worst case, your mother could always buy a Steam Gift Card and give you the code for that)
Hopefully someone with more expertise in cross-region stuff can help you out, as I'm far from an expert.
Well considering I can see the game on the store and the price is the same I don't think the region will be an issue.
Your mom can set up her own Steam account, and send the gift to you via e-mail and that should work just fine. She won't be able to send the game "directly through Steam" because that requires you two to be Steam friends, which is one of the community features that you can't access until you own a game on Steam.
Gifting a game shouldn't be restricted (unless it's specifically disabled by the developers) only trading was gimped by the new restrictions.
Does your mom live in Eastern Europe, South America, or South-East Asia? If not, you're most likely fine.
EDIT: just re-read and you said your mom was in the states, so it's all good.
Ah thanks! But I live in SEA..but like I mentioned earlier in this post the game pops up on the regional SEA page for same USD.