Would you be happy if, having pooled your wife's and your own miles into a Family Account, I was somehow able to "use those miles"?

Just because you are related to people in a completely separate family account, you cannot transcend the rules relating to Flying Blue (Family) accounts. You cannot touch/use anybody else's (Family) account. I am just as much of a "stranger" to you, as you are to your Mom/brother (from the perspective of Flying Blue) - so the expectation that you can "use" their miles means I should similarly have the same expectation to use *YOUR* miles!!!! (OK, granted, you have a better chance of getting to know their FB account numbers than I am likely to be able to find yours, but even so, it doesn't change the overall logic. You are not "family" to them, in FB terms, just as you and I are not "family" in any terms)

Being a blood relation in no way changes that - you have no connection with any family (in Flying Blue terms) other than the Family Account you yourself participate in. You *cannot* use miles in somebody else's account; you can only use miles in your own (Family) account.

(Family accounts are new; they only came into being 4 months ago. I am not sure if, having a Family account, you can issue reward tickets in the name of third parties outside the Family, which is possible for "normal" Flying Blue accounts. In theory, if your Mom and brother pooled their miles into their own family account, they may be able to "buy" an award ticket for two of you, while you "buy" an award ticket for the other two of you. Splitting your family into 1adult+1kid (for one such ticket) and 1adult+1kid (for the other such ticket) may be the way to go there. But it opens up the possibility that you cannot all travel on the same flight. It also leaves you needing to buy an unnecessarily expensive one-way cash ticket to get everybody back home.

Again, I'd advise keeping the miles for some other trip - when it's just the adults travelling, perhaps)