Sorry for your loss.
At a local school they have just planted a memory tree for a young student who passed too soon - the tree is in the play area.
Sorry that I have no answer for a tree in zone 5b that could start in a pot and be moved later - and I would hate to think of the emotional impact if the potted tree failed to thrive, but I have an idea.
There may be places that you can plant a memorial tree in your local area. Or you can work through the Arbor Day Foundation and a tree will be planted (somewhere). I don't know your exact location so cannot be more specific about where the memory tree could be planted but here is an example of one such program.
Later, after your SIL moves to her new home you can buy a suitable tree for her new yard/garden.