MIL and SIL visit help

[vanillacourage]

Honestly, this is cheesy but if she's typically a nice person, then if it's just clear that you make her son very happy in the life you're creating together, I think you're good.  That + the basics of being a good host (have snacks & drinks ready to offer, etc) and you're set.

[hopecounts]

Agree with vanilla, do normal cleaning (mop/vacum/etc) and be welcoming with refreshments and what have you and you should be fine.
Fresh flowers around always make a place seem homey so maybe some sitting in the den if no one is allergic. oh and use the realtor trick if you'd like of having baking smells preferrably something cinnamon either from baking or scentsy/pinterest recipe for on the stove home scenter/etc.