Is it odd that 2 weeks after my father died, my sister tricked my mother into changing her will?

You should get your mother to make changes back to what it was if its what she wants. Have you spoken to your sister and asked her about this?

It isn't really odd for your mom to update her will and POA after the death of your father, and it could have been convenient to do so only a short time after his death simply because they were meeting the lawyer anyway to settle his estate. Depending on your age, it may be a good idea to include someone from a younger generation as POA and executor, is there something untrustworthy about your niece?

As already stated, your mom can make changes as often as she wants, but perhaps it would be good that she gets legal and financial advice from a disinterested third party.

Well, if your mother is thinking straight now, this is easily remedied. Mother can simply make a new will or make adjustments to the current one, to reflect what she really wants. Same wiht DPOA and Healthcare Proxy. She can ask her bank and her lawyer why they weren't looking out for her interests.

But it won't be hard to fix things, now that mother is her competent self again.