Cousins of whom, though? Anya Waynwood seems to have been Lady of Ironoaks or at least the rightful heir of it before her marriage, but the only thing we have to go off of is that her husband was of a less noble house or no house at all. However, we know of Anya having potential male cousins and nephews, so perhaps she had her Waynwood children before she inherited the land. Inheritance is a bit bizarre in ASOIAF, so it's unclear who's house inherits after parents' deaths. It's a bit bizarre, though, that the only known children of Rohanne in the books is her children with the Lord of Casterly Rock, so either she had a child with Eustace who inherited the lands or a relative came in. What's really strange, though, is the inheritance of Standfast. On the wiki, Standfast is regarded as still a holding of House Osgrey, while Coldmoat was their former hold, but it's hard to say for sure on both accounts given the inaccuracies sometimes found on the ASOIAF wiki. Sworn Sword ended with a sort of optimistic and changed way of life for Eustace, and he was a particularly family-oriented person, so either he had some serious plans of copulating (which could have easily happened and we just aren't told, he did have four children who survived to adulthood), or he accepted that his lands would fall to House Webber and thought he'd at least enjoy the hot new wife in the meanwhile.
If that line died out and no other Osgreys were there to claim the titles, it's a bit bizarre we haven't heard of the Osgreys at all, so there's no solid confirmation of them being around or not. Maybe it'll come into play one day. I'm sure House Osgrey is just GRRM showing what stubbornness and honor and loyalty can sometimes bring, and it would be against that metaphor to give House Osgrey a particularly happy ending, but it's so hard not to fall in love with Eustace Osgrey and his house's history.