re: So how do you approach your parents about their estate plans?

Prefacing my comments by saying I just went through a lot of this as my dad was killed in an auto accident.

I'd tell them that for their wishes to be upheld, they need to create a trust if they have not already. The trust needs to be for the children specifically. Their will needs to state that a certain percentage of their assets go directly into the trust.

This way, your sibling can't contest it.

Good luck. We had a lot of luck by being open and honest, and having uncomfortable conversations proactively.