I was thinking there about how my BIL maintains that all families are a little bit weird to someone else.
Thought he was being flippant but then I remembered that my in-laws keep their family toothbrushes and toothpaste in a drawer in their bathroom. If you need a bit of floss after Sunday lunch you need to stick your hand into a sticky, damp mass of plastic and bristles.
What slightly weird things do your in-laws do?