Mom is obsessed with trying to touch my pregnant sister’s stomach

Me (31F) and my pregnant sister (24F) are pretty different people but pretty close. I do not have any kids and have never been pregnant. She’s pregnant with baby number 2. We both have tumultuous relationships with our mom, but her relationship is even less civil than mine. During this pregnancy I can see my sister making the effort to put aside her own feelings about mom, to make sure our parents get to have a relationship with her daughter, but honestly it never seems like enough for mom. My sister is pretty firm about her boundaries with her, and it’s almost like that makes her even more pushy. Last night we had her and my parents over. At one point my mom was speaking lowly to her. Come to find out later mom was pleading with my sister to hug her belly when my sister might be “feeling up to it”. My sister ignored her, as there is no part of her that would be “feeling up to” mom up close and personal with her. Apparently this is not the first time this type of interaction has taken place between them. My sister finds this very off putting, and I get that. But Being I’ve never been pregnant, some of the nuances on things have been lost in me, honestly. Is it normal for expecting grand parents to want to touch/hug/ect expecting mother’s bellies? I know none of this is up to me in this specific instance but it got me thinking. sometimes I have a hard time knowing when mom is being emotionally manipulative and when she’s just being…more like a normal person? It’s like everything she does rubs me the wrong way and I’m starting to feel nuts.