Family Matters: I don’t Want to move in with My Sister

My sister is pressuring me to move in with her, how do I get her to stop?

My living agreements right now are unusual: I share a 2 bedroom apartment with my ex-boyfriend ‘Kyle’ and his girlfriend Jen.

Kyle is my ex because he cheated on me four months after we signed the lease.

He moved Jen into the apartment one month ago.
Kyle is dead to me and I don’t acknowledge his existence.

Weird thing is, I actually get along with Jen really well and just remind her that Kyle cheated on me so, he’s probably going to cheat on her.

Economically, I think is best to just ride out the lease.
The trouble with my sister ‘Kristie’ started last week. She and her husband offered to let me move in with them in their 4 bedroom apartment for much cheaper rent. I declined, mainly because my sister had triplets earlier this year. Her social media is full of talking about how hard it is to be a mother.

I’m afraid she’s going to want me to help extensively with the babies. I don’t want to live in a house with three six month olds kids and really don’t want to be an unpaid nursemaid.
Since then, I’ve been getting passive-aggressive comments on social media and email from her, her husband, and my parents about how my living conditions must be awful, how hard my sister’s life is and she could really use the help, and how ungrateful I am to my family.

About writing out the lease, I think I will execute the plan once I find out 3 of us okay with the idea and I will look for my own apartment thereafter. My fear is that my sister and her husband might turn me to an unpaid babysitter. I feel things will get tense between me, my sister and her husband. Or what do you think I should do?