Got to staffs m and f want another f worried about fighting will this still happen if i get the older f done?

my female is the mum of my male and they get on wonderfully i have just seen a lovely staff i really want its the sister of my older ***** who has the pups im concerned if i was to get this other one there will be a lot of fighting which i dont want i will be gettin the older ***** done as i have a male and dont want mother and son to mate i know this will be a prob keepin my male and the new pup which is female apart for a while untill i decide which other one to get done also could you please give me as much info about the situation with f f and m and also wot the score will be if the two younger staffs did mate bearing in mind my male has the same father as the ***** i want i love my dogs and never had a problem with this breed could you please help as i dont want to get another pup and have to rehome her as its not fair once she is settled my other 2 staffs are freindly loving dogs with lots of children around them all the time i dont cage them when im out either as they only play