if you choose to go ahead and do the shopping i can offer a suggestion for 'the little darlings' and mil-
give them each a 'now and later' gift-idealy the 'now' gift is one of those loud rather tinney sounding and annoying animated holiday plushes that plays only one line of a holiday song over and over and over and over (like 'deck the halls with boughs of holly, deck the halls with boughs of holly, deck the halls with boughs of holly.......'). make sure it's the good inexpensive type that has the unreplaceable battery stiched inside so it can't be removed and their parents and grands can enjoy listening to it for hours and days on end (bring a set of inconspicuos foam earplugs for yourself

). since your sil sez 'no gift cards' do the next best thing-go to chuck e cheese or whatever loud, annoying kid themed arcade is near by and buy each child however much you wish to spend IN TOKENS. put the tokens LOOSE into a gift bag with a holiday greeting that says you know how much they will enjoy spending a WHOLE DAY at the arcade with mommy and daddy

for mil why not give her some gifts she can use to have 'quality time' with those darling girls? gifts that come with a card that says on the front "read me outloud before you open this gift" (the card reads something like "special gifts that have to stay at grandma's house for special times with her SPECIAL girls")-i would suggest these gifts might be any or more than one of the following:
a snow cone machine,
a cotton candy machine,
2-10,000 bead kits
multiple tubs of 'floam'
or better yet-a karoki machine with LOTS of kid's bop type songs the girls will love to perform over and over and over and over and over JUST FOR HER.