Actually, that's exactly what I would do. Why waste the risky option of revoking her titles, and upsetting other vassals, when you can quietly marry the younger sister, get some babies, and then murder the older sister and her family?
Did that more than once in CK
Eh, I'm still not sure who qualifies as kin when it comes to being a kinslayer, so I tend to refrain from murdering my in-laws. I really hate being a kinslayer.
Although murder and inheritance are viable options, certainly.