it's shorter than saying former sister-in-law and also defines their relationship as current continuity has it.
I think Marvel would like it to seem to fans that Lorna and Pietro were on X-Factor at the same time so it makes sense that he'd introduce his wife to his sister. Except at that point Pietro and Lorna weren't siblings.
It's also possible that Inhuman law may say that the end of a marriage doesn't end the familial connections.