I know these are morbid comments and not exactly what you asked, but it's the kind of thing that people tend to avoid because it's uncomfortable, but which can cause so much pain for those left behind at the worst time to deal with bull****
Something you'll definitely want to have in place which no one likes to think about is iron-clad wills for all three of you, outlining exactly what will happen if something happens to you and your husband, or you and Z, etc. If you and Z are killed in a car crash, you want to make sure Z's parents can't take your kids away from your husband. So then it's really important for Z to have a will saying that he wants the kids raised by your husband. If you and your husband die, you want to make sure that Z won't have his home taken away from him (assuming he's living with you).
Gralson: my husband (works out of town).
Auto: my girlfriend (lives with her husband Zoffee).
The most dangerous phrase in the English language is "we've always done it this way."