Can my sister get a POA over my dad without other siblings or my mom's (his wife) consent?

Only Dad can name who he wants as his Power of Attorney. Since he is married, usually your Mom will be named first POA and if she is unable to make decisions, then Dad's POA should also name another person, usually a grown child as 2nd and 3rd if the siblings are usually on the "same page" when it comes to decisions.

Let's please remember that we don't get POA "over" someone, but rather "for" someone.

If dad is mentally competent, he can name anyone he chooses. He doesn't need anyone's permission. The notary would know if he was capable of understanding what he was doing.