OK Sionnsar...
My son in-law’s aunt (his father’s sister)
will become my son’s mother in law.
But I just got home from a week on the East coast (it's 1:30 AM body clock) and am on deck for the church service tomorrow (with the Archbishop!), so I'll try to come back with the diagram tomorrow.
This one doesn't seem to actually lead to lots of confusion, if I have charted it correctly.