I think it would be funny to tell my nieces their grandparents are incestuous.
What motivates you to tell your nieces their grandparents are incestuous?
Because I love my nieces and I care about them a lot.
Insinuating to your nieces that their grandparents are incestuous is dishonest and not credible. If you love your nieces as you say you do, you will not tell them lies.
I don't care about what you think about me. I'm going to tell my nieces that their grandparents have incestuous relations.
There is literally no reason to hurt them like that. You shouldn't have a relationship with them if you only want to cause them harm.