To be honest, this is why them repeatedly mowing over the clearly marked line(despite being told otherwise) really concerns me. I don't know how much of a leg they would have to stand on, but I'm afraid that by mowing it they could claim they were maintaining the land as if they lived there and could claim it by adverse possession.
Of course, the land is also maintained(mowed and otherwise taken care of) by the person who hold the deed and people acting on her behalf(namely her dad and I) so I don't know how much that claim would stand up, but it's something I try to be aware of.