They've been referred to as brothers. Not sure if it comes from the fan base or some merchandise but I do remember that being a thing even way back.
The original release of stalker (series 5, not the one who glows in the dark) mentions "Brother Boar" so there is an implied connection given there is a Boar among the group. I don't think anything was actually said about it before the re-release had them being family.
It's probably something that the community did and Neca and the I.P holders ran with it.