What a bloody hassle. As you've said you are a nanny and part of the job is to multi task why would Grandma think that you would have been hired if you couldn't do the job. It's easier looking after two children when they're both sick as you can chill at home. It's harder when one is sick and the other is full of energy and going stir crazy having to stay in. I think you need to put your foot down with Grandma before things get worse. I have friends who've had to put Grandmas in their place as they keep coming round and taking over. Your boss should back you up because if it's her mother in law I'm sure she's had issues with her too in the grand old style of trying to get along with your partners mother.