Actually, a roseprincess makes some good points, albeit a little more harshly than I would have.
You cannot control other peoples' behavior. You can only control your reaction to it.
Yes, your MIL and grandmother were wrong on many levels. But I am not sure why you are giving them the power to have such control over your fellings and emotions. I mean, look back on what you have written. You have allowed them to ruin your birthday.
You have changed the locks...good start. Make sure that neither you or DH gives anyone the key.
Change any other access points to the house that they may have, such as remote garage door openers, push button lock combos and so on.
Don't hide a key anywhere outside, where they may find it.
If you give a key to a trusted neighbor, tell them that under no circumstances are they to let your MIL or Grandmother into your home if either you or DH is not there.
Get caller ID and limit the amount of time you speak with them. DH has a "difficult" sister, and caller ID has been a godsend in terms of speaking with her. She leaves messages and we respond at times when we sort of "know" she won't be home, or we respond via e-mail.
Figure out how you will handle conversations with your MIL and grandmother when they begin to take a turn that you do not like. It could be as simple as "Gram (or MIL), I am not discussing this subject with you. Thanks for calling and I will talk to you soon". And then hang up. The key is to keep your voice calm and neutral. It's called limit-setting and it works. My parents had this habit of getting into bickering sessions when I was visiting them until the day I said "I am not going to listen to you two bicker". I kissed them good-bye calmly and got up and left. The next time I visited, and the bickering started, I said "I am not going to sit here and listen to you two bicker" and started to get up and leave and they were like "no, no, stay." and the bickering stoppped. It just takes retraining and limit-setting.
Yes, your Grandmother was good to you growing up, and your MIL is essentially a good person too. That does not give either of them the right to run your life or interfere, and they need to learn that. It will take time and consistency, but they will learn. You have probably let them get away with it in the past, so they think it is an acceptable behavior.
You will probably also need to figure out other child-care arrangements, because they will try and "punish" you by not agreeing to watch your children any more.
You will also need to stop involving your sister and other family members in your disputes.
It's time to put on the big girl panties and deal with this.
Good luck.
(I'm looking for the Tink ones myself. 
Couldn't find them at my local Target!)
