That's lawyer talk, not standard English, in case you thought it was. "Last of" is strangely embedded in there, as in the last of the Mohicans. They mean the last person still alive of a group defined as me and my beneficiaries (whichever of those beneficiaries are alive when I die).
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.