my girfriend and i are engaged and during the year of 2015, does she qualify to claim my sister if she supported her?

Probably not.

As your sister, she could only claim her if a) she lived with her 365 days in 2015; b) she can prove she supported her; c) no one else--including her parents and you--has a superior right to claiming her as a qualifying child.

Your fiance is just that a fiance. you aren't married yet so there is no relationship between her and your sister.

did your sister live in the household of your g/f the ENTIRE year, did your sister make $4K, did your g/f provide more than 50% of your sister's support and can your sister be claimed on another tax return, these are the criteria for a qualifying relative who doesn't have to be related to be claimed

Usual rules always apply. Your sister might qualify as an "other relative" which is someone who does not need to be a real relative.

Here are the qualifying child/qualifying relative rules:

https://www.irs.gov/publications/p17/ch03.html#en_...

https://www.irs.gov/publications/p17/ch03.html#en_...

What as some sort of prize?