1. I believe your sister are already a residence in NZ? if yes, she can only sponsor your partner as a visitor and the maximum he can stay in NZ is 9 months. Even though you already in NZ, it won't affect his application at all.
2. Yes he can. To apply for a visitor visa, Generally, INZ will require financial evidence for about $1k/month to stay in NZ and he's not required to provide this if your sister can financially sponsor him. This is just the point that differ if you can get sponsorship from a NZ resident. Otherwise, as stated by JandM he can apply for his own ES visa in his own right.
more info about sponsorship:
https://www.immigration.govt.nz/new-...table-sponsors