Marrying ladies of executed lords

[Leonard]

I was roleplaying and wanted to have margaery tyrell as my spouse (I read somewhere that if you execute renly you can marry her). So after that i had to fight very long wars and after i made peace with the stormlands i tried to marry her it still says she is married to renly even though i executed him. I have very positive relations with her (tourney grind) and i can even ask her to improve relations with(dead) renly. Is there anyway around this? Or is this just a bug?