I don’t agree with the majority actually.
From your PIL’s perspective, their wealth will primarily be for their son and grandchildren. IMO they should also want to do right by the mother of their grandchildren, but when we’re talking private jet wealth, that needn’t be half (on divorce) or all (on death) of DH’s inheritance.
I also don’t see this as interfering. How can they be meddling in your marriage? This is about their money, not money that your DH has earned
. You say you’re both doing fine already; your PIL’s know any inheritance will be bonus money, not food-on-the-table money.
They are your husband’s parents, your children are their grandchildren. They can’t take this money to the grave with them. They will want to see it put to its best use. That will invariably mean “left to my family”. Divorce, death and marriage are a fact of life. There’s a good chance you will walk off into the sunset with a sizeable share of what they’d like to see benefit their son and grandchildren.
In their shoes I’d set up a trust for their grandchildren, with your DH and another family member (not you) as trustees. It’s all they can do. They can’t ringfence the money from you once they’ve given it to DH, as you’re married.
TBH, if they’re that wealthy, I’m surprised this wasn’t all planned for in advance. Sounds a bit off to me...