My MIL watches DD and for the most part she is very good....I gave her the green light to start practicing with sippy cups with a little bit of water or BM. I just found out that she has been giving her juice in the sippy and I am beyond pissed! She never asked me and to me that is a big deal. Our pedi said no juice at this age. I assumed it's because of the sugar but I never asked. Am I overreacting or do you think that I have a legitimate concern? She has been doing it for the last three days now and it would prob explain the poop explosion she had last night. My MIL is very sensitive to any criticism and she watched DD for free so I have to be careful, I feel like I am in a tough position....WWYD?
Re: Juice at this age
It's your kid, it's your rules.
My almost 3 year old rarely gets juice...I just think it's empty calories.
I would not be happy if someone gave my baby juice at this age.
Married: 5/12/07
DD: 7/28/10
TTC#2: 10/2014
It's really up to you. I give mine a little juice every now and then but I make sure its very diluted.
DD - Juliana Joan - Born October 27, 2010 - My Little Princess

BFP 1/14/13 - M/C 1/22/13 @ 5 weeks
BFP 3/20/13 - EDD 11/11/13
I've given it to Gavin a handful of times because he seemed constipated, and it was very diluted with water. Now he only gets water in there.
However, it is your child. Take a sippy cup over there filled with water, and ask that she only give him water or bm. I don't think it is a criticism, and hopefully she is not offended.
Let your MIL know that the pedi recommends no juice and the reasons why.
She probably just has no idea.
My biggest issue would be that your MIL gave juice without asking you. It sounds like she has good intentions, so like a PP said, you can send the cup with water in it, bring up the pedi's rec, or mention that you're holding off on juice because it gave her diarrhea or something.
Mostly, it's your baby, so she needs to keep you informed and do what you wish with your child.