With the mother in law - surely suggesting the addition of a couple of salads - which you'll happily bring - shouldn't go awry. In fact, in any social situation where you're a guest, offer to bring a platter of vegies & dip or a salad - at least you'll have some control over what you're eating, particularly if you're making a part of it.
in terms of alcohol, you could try the pure blonde or low cal/carb beers, if that's your thing. or you could drink soda water with bitters, or lemonade with bitters, clear stuff is easy to pass off as vodka, particularly if you have say 1 vodka at the start of the evening, and then maybe one more a bit later. no overkill, and no hangover the next day.
i can guarantee you that probably 90% of people attending these functions will be in some form of self denial or diet or have food allergies or special considerations - some form of dietary needs - that needs to be attended to. so do not ever be afraid to say to a host that hey, hi i just wondered if some salads could be provided on the day, or i'm happy to bring them. if need be say you're diabetic and can't eat greasy overkill food - with diabetes people do listen to what your specific requirements are - or go vego like me - i always discuss ahead of time with any invites about that stuff. i think these days it's just normal to take into account people's differing needs.
and let's face it - for those of us who are carrying the extra fat - asking a host to please have something that meets our diet/meal plans to some degree, is not going to be seen as hardcore. they'll probably be happy to support you and your goal. you don't need to make a huge fuss - just plan ahead, ask, and if need be, bring your own!
cheers
kaz x