why was the son in law alone in a room with his mother in law?

Question

that just doesnt happen

Answer 1

I thought you were telling a joke—sure it happens often and innocently in my experience..

Answer 2

He may have been asking her how to surprise the daughter, do not jump to conclusions,

If you think something fishy is happening keep an eye on their body language when around each other, this will tell you more than you would probably wish to know, because if it turns out they are up to no good what are you then going to do with the info

Answer 3

happens all the time...
but you seem suspicious.
there are lots of reasons it could be legit.
but you seem to think in this particular case something is going on.
ask them.