Gross relationships

I have been enjoying my comfort save for a while. No issues until today. 

I'm a console player. No mods.

My sim and her spouse got married. No issues. Went as well as wedding stories can be. I get them home to woo hoo to realise they can't so I check their relationship IT SAYS THEY ARE SISTERS. This sims met in University. No issues. Then they are suddenly sisters?? I delete her spouse and replace her with a previous version of her in my gallery AND IT STILL ENDS UP MAKING THEM SISTERS???