well u r right.it is her full right not to tell me anything.but then it is not my duty to take care of her son and she cant expect that just by mailing only my husband and not asking me she can just come and disturb my plans I had planned a birthday party for my hubby that is now cancelled cuz of her interference.
Then just tell her nicely that you're too busy that day, and can't take care of her kid. Don't lecture her (your not her mother) or complain to any1 abt her; just tell her you can't do it, you have other plans.