Mate, you could have just parked to the side of their house, behind where their car is at the moment.
No need for drama, and tbh it doesn't seem like a big deal that they parked where they did.
Sure, it's not ideal that they pulled up directly behind you, could just be a miscalculation or them not thinking at all. They certainly could have backed up further to the side of the house, but you can't really be angry at them for parking where they did.
A single strong wheel lock right and you're out of there. I think you're bringing the drama upon yourself by parking in front of the their house to begin with when there's plenty of neutral parking space both to the left and right within the photo already, let them have the space in front of their place when there's bags of room within 10 seconds walking distance in either direction.
To be fair I wasn't even aware they owned a car until tonight, when my girlfriend told me whos it was. That's where I've always parked for years when the in-laws are up. There's room over the other side if its a problem, but parking behind my car like that is clearly is just being a awkward chewy **** when they don't own the road. It's literally the first time I've parked there i 4-5 months.