Good advice all around!
If the kids are around during the startup of PDA, quietly send the kids to look for dubloons out in the back yard and then speak gently with your sister.
In any case, I would suggest along the lines of this:
(I am happy you feel loved. But you know we were raised to believe premarital relations are wrong. The kids are getting old enough now to ask questions and I don’t want to have to tell them you are doing something they are taught is bad. If you could refrain from pushing that issue in their presence it would help things tremendously. This isn’t something I want them to have to deal with at this age. They aren’t ready for it.)
If she refuses, because she has an agenda, (which she must… because I know very few heteros that make a point of PDAs when they are around family), then leave the kids with dad. And when they comment they never see the kids anymore, just say it’s better that way. They were getting confused by Auntie and her girlfriends antics.
Don’t let it become a battle about who is right or who is wrong, or that you’re keeping your kids in a bubble or whatever… state your position. (Always do it with a smile on your face. It is physically impossible to smile and raise your voice at the same time. Forced smiles work too.)
As for mom… she needs to be retrained. A manipulative person is someone who can make you feel guilty for the fact they abuse you.
Nip it! Nip it!
Ask mom to babysit. Tell her you need an answer by Thursday. If you have no answer, make other plans.
You set the boundaries though. Then if she gets mad, you can tell her “I needed an answer by Thursday. I told you.” That puts it back on her. It’s a game of emotional pingpong. Just keep quietly knocking the little ball back to her.
It’s HER issue to jerk you around. Don’t catch the ball and hang onto it and start hitting yourself with the mallet. That’s not how pingpong is played!
When she tells you your children are horrible in front of your children, smile and say “Now, mom, is that the memory you want them to have of you?” You haven’t attacked her. You haven’t ordered her out of your house. You have reframed the issue to how she is making them feel. Then she can look at them. And they will look at her with that “Well…?” look in their eyes. And she will have to either say “Yes! I want them to remember me as a hateful old biddy who never had anything nice to say about them” or “NO, I love them! I’m sorry!” Or something in the middle.
But again… it keeps you from having to absorb the punch and let her criticize your children (which is a direct attack on YOU, my dear, whether you realize that’s what she’s doing or not). When she does that, if you don’t defend your children, they notice. If you do, then your mother can accuse you of starting a fight. She is the queen of passive aggressiveness. And it may never even dawn on her that many grandmothers don’t say their grandkids are awful. (Some refuse to even THINK it! )
She may have been raised like that and it’s all she knows. Break the cycle.
The best way to short circuit people is by asking them a question.
Grandma: Your kids suck!
You: How do you mean that, Mom?
Grandma: They’re rude and awful!
You: (Looking at the children.) Hearing you say that is hurtful to them. Is that how you want them to remember you?
You can win this! Life will be better. Because you know what? Wrestling with a pig just ain’t worth it. You get dirty and the pig likes it. And that’s what passive-aggressive people live for. Emotional wrestling.
And if people who treat you badly avoid you when you don’t give them permission to mistreat you, what are you really missing?