The father can sponsor the daughters husband (your brother in law) cuz He don't need the 10000 requirement as it is a visit visa not residence visa , and it is his right to try. If you or father apply it will not affect sister cause she has residence.
They either accept it or not. But again it's not going to do anything to your sister's residence, she has her sponsor legally
If you still doubt what I say, just go direct to the immigration dept and ask these questions to get it from the source.
The father can sponsor the daughters husband (your brother in law) cuz He don't need the 10000 requirement as it is a visit visa not residence visa , and it is his right to try. If you or father apply it will not affect sister cause she has residence.
They either accept it or not. But again it's not going to do anything to your sister's residence, she has her sponsor legally
If you still doubt what I say, just go direct to the immigration dept and ask these questions to get it from the source.
Best wishes