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.
http://whitehouseoh.gov/govern...
Later, after your SIL moves to her new home you can buy a suitable tree for her new yard/garden.
