Marriage

So my current heir is married to a woman named Gemma Forger which is the heirs 2nd cousin 1x removed. They have 2 alive kids and another one of the way They are both young adults. Gemma has a sister that is a red head her sister is blonde. Iris III (Gemma’s sister) looks a lot better so it would help the future generations and it would bring red hair into the family. My problem is that I like Gemma a lot but I feel like Irise the 3rd would bring better to the family. I don’t want the heir to divorce the Gemma so she has to die somehow. So my question is should I kill Gemma and marry her hotter sister or should I just leave it. If I kill her give me a good story line to bring into it.