This is of great interest to me! I have 2 DBs, 1 a millionaire in his own right, the other not, DH and I live modestly.
Non-millionaire DB has had a few failed enterprises, DPs gave him large amount of money towards a house. Before they did so, they explained to me and millionaire that when the last of them dies we would have equal amount each (with inflation) to DB first from the estate, he gets nothing apart from the remainder divided by each of us. We were asked to agree, which we both did.
Suggest you speak to them both and find out exactly what the situation is, keeping in mind he is not obliged to treat you the same if he does not wish to. I say that he is not obliged because currently my son will inherit all from our estate and our daughter nothing, owing to her drug addiction and drug addict partner. She has been quite evil toward us, only nice when she wants money, lies, broken family etc, etc. She doesn't know yet, shame I won't be around to see her face!