Last year I drew up a chart of all the technical committees I was involved with [at the time], my involvement with each, and all the other related committees and their "lines of communication." It was a pretty impressive, even daunting, diagram but the engineers who saw it just went "Hmmmm..." and understood what they'd never grasped from verbal communications.
I started to draw a diagram but realized I didn't know the particulars of your daughter's husband's relationship to his cousin.
The traditional English expressions of relationships tend to break down when more than one relationship exists. For example, I know of two sisters, the younger of which (stop to think) is also aunt to the older; the same situation produces a daughter who is sister to the mother. [Situation: grandmother adopted one of her granddaughters.]
OK Sionnsar...
My son in-law’s aunt (his father’s sister)
will become my son’s mother in law.