Actually, the relationship is neither because the two are independant. Membership numbers identify members, and there is a unique membership number for each member. Branches have no formal relationship to members, so a branch can exist with no members, all members, or anything in between.
The relationship (showing members belonging either to or in branches) is separate from both memeber and branch, and the relationship is not constrained by either member or branch. This makes it a many to many relationship (which SQL represents as a pair of 1 to many relationships).
-PatP