how do I stop myself from training fil's dog...

[hatwoman]

fil is here with 12 week old pup. and I'm having to stuff my fist in my mouth. he's had dogs all his life so I really really can't wade in. but it's hard...

Tbh, if it was me whose pup it was, and it could be, as mine's that age, I'd be begging you to train the little bugger :o If I say pretty please?

You don't. MIL bought a dog and was training it when she got ill, when we went to stay FIL hadn't done a thing and it was going feral so I got a treat bag and started training her for him under the guise of playing/walking her.
Working on weekend visits we got her walking to heel, returning to call (that took awhile...) and to stop jumping, and then said
'OOh look, if you do this, she does that!'

I shamelessly used DD's as well, encouraging them to get her to sit, fetch and drop things for them, again, as far as FIL was concerned we were just playing in the garden

MIL's previous dog was so badly trained she used to try to bite me if I kissed DH in front of her, or touched MIL.

(these were toy poddles so I could easily stop her though, she was not allowed loose near the children due to bad training, hence me insisting MIL trained this one properly)