The message
from Sacha contains these words:

FIL is 'father in law' - more intertnet shorthand. Sorry to have muddied the
waters!

Might as well tell him the whole set...M(other)IL, S(ister)IL, B(rother)IL.
One's partner, squeeze or whatever is known as SO, Significant Other.
For women, your SO might be AKA (also known as) your DH, Darling
Husband, unless the SO of the DH is American, in which case DH is
probably a Darling Hubby.

HTH

Janet