The most important factors are that they are healthy and that they are getting on with each other.
Character compatibility is the most important factor of a successful long term bond and while more boar pairs make it together than not, being siblings and getting on as babies is not going to guarantee it, but it’s the best you can do when buying piggies from a breeder or a pet shop.
I would encourage you to go to a rescue centre to find some piggies as that way you can guarantee that they will be healthy and aren’t harbouring any illnesses and that they are well bonded and you won’t encounter bond problems down the line.
You must also check for yourself that they are in fact boys as missexing is common.
With boars, ensure you have a big enough cage - 180cm x 60cm is recommended to give them enough space and not create problems between them as teenagers.
A Comprehensive Guide to Guinea Pig Boars
Dominance Behaviours In Guinea Pigs
Quick Information Bundle For Wannabe Owners