hi barbara; this is so hard, as everyone pointed out the MILs are so invested in insisting thier way is better than yours. i hope i won't have to become a MIL to find out why, hopefully i'll remember what it's like to be on our mom-end of this.
but i would say evaluate the real damage of the problem and the frequency against the positives of your relationship w her; if she's otherwise great, and it's not constant, try to let it go. as you will see if you have more kids, schedules tend to go out the window anyway.
but if there's a pattern of her insisting on goofing up your routine, you need to be the one to say it to her, because she'll always go around your husband or ignore him when he's out of the house. chances are unless she's a real psycho, being straight with her will embarrass her enough to get her to get on board with you.
the key is often finding clear words that you feel comfortable with and that are assertive, and also striking a bargain can help; maybe something like;
"I know you really love me and the baby and you want to do what is best, but this feeding schedule is very important to me, and i need your help and support with it. Would you feel all right about giving me a lot of input on something else that i need more help with, like planning the baby's summer clothes / finding good prices on diapers?"
something like that?
good luck!
j