I expect your case is one of a sibling not having parents properly married and thus you need to look at the relationships in the profiles concerned.
However, oftentimes there are subgroups which might have a surname you're interested in but don't connect to your tree, or any other trees. WikiTree connections do include marriages, but as Anne says, a group of people need to connect with a member of the global tree, which more than 8 million profiles are already connected to. Putting these facts together, a good way of joining your group to the Global tree is to trace back the ancestral lines of spouses in the group. That will bulk up the size of the group and give you more families to make connections via. Eventually you'll make your first connection and after that they will come fairly regularly (In my experience.) I think it's more fun to find connections myself, but if you're anxious to get that first connection. to a subgroup, feel free to post a Wiki ID and ask for help.