Family tree is broken

My sims family tree is broken, members of the family that should be there are missing. My family tree isn't very big, so should have no issue finding them, but are completely gone. 

As highlighted on the picture, the sim is missing links to her son, therefore any family tree connected to the son has gone too. 

Does anyone what this is, as I don't want it to happen to other generations on this tree and any future ones to come.