It's not as straightforward as you've put it.
Boars can very well live happily together as a duo or even trio, provided they compliment each other in characterand/or age, and have PLENTY of space. With oodles of spaces, they can even form groups of twenty bachelors and have no fights. Generally, the chances of a fallout between boars are bigger than with sows, especially during the hormonal phase. Neutering does not change the odds.
Neutered boar and sows(s) usually works very well and is probably the most stable of all relationships, but you have to pay for and risk an operation.
Two sows can still fall out even after spending most of their lives together as sisters. Some older sow pairs can get rather crotchety.
But the big no no is putting any full boar over three weeks old with sows (including sisters) and leaving an unneutered boar in at birth.
Any pregnancy is very hard on the mummy, and as lovely as babies are, they are not worth the stresses you are putting your poor sow under; especially in back-to-back pregnancies.
http://www.guinealynx.info/breeding.html
This is the reason why we stricktly do not advocate breeding in any way on this forum. Please read the rules!