Happens also when travelling to the Future. Two of my elder sims, Natalie and Colin, a married couple, died there on the same day. Their children only show as children of Natalie and all their grandchildren disappeared from the family tree. They still show as children to their mothers and fathers in the relationship panel. After that, the family tree turned completely unusable, doesn't even open. I would like to at least see it.
Also, the gravestones of Natalie and Colin disappear if I try to return them in the past. Any fix to that?