I read in Dr. Sears that you can spoon feed them breastmilk and they will sort of lap it up? Or that you can give it to them by syringe. It would utake patience from your MIL though. That's the only thing I've read. I just know my doctor is on the very strict side and says NO LIQUID in a cup until 15 months because there is a 20% chance of early weaning if you give it? Ds is only 5 months so I am NOT speaking from experience here
I do know that a woman at my LLL meeting couldn't figure out why her 8 month old was "weaning" himself. She had been giving him sippy cups of water (not the same thing, but thought you might want to know?) I just know it would break my heart for ds to wean early so I'll share that info. Good luck with figuring something out.