You have two options, while you wait:
1) You could create profiles for your brother and grandmother, and continue working on the tree, and then when you're added to the trusted list of their duplicates they can be merged together.
2) You could create profiles for, for example, your grandmother's parents, not connected to your tree, and connect them later when you have access to their profiles. To add someone without them being connected to your tree, go to Add -> New Person in the top right corner of the screen.
~Lianne