Hmm, this is a very very difficult situation. Do you know the approximate date of uncle's arrival? Can you afford to take your mother away for a few days around that time, so they arrive at an empty, locked-up home? Or send your mom to visit someone uncle doesn't know and cannot reach for a few days so that, again, the home is empty and locked when uncle comes?

Hehehe, if it was me I would turn off the lights and just pretend not to be home, like on Halloween but your mother is much nicer and less selfish than I am.

Keep us posted.