12 years ago when I was in your situation, I continued nursing my toddler. LLL's Breastfeeding Answer Book lists it among drugs generally considered safe when nursing.
My doctor had already told me to stop nursing, though, before I went onto terbutaline, and I wound up delivering at 28 weeks. There was never any clarity about how the factors influenced one another.
If you're sure nursing isn't increasing your contractions, I'd say go for it. It'll help keep you and your older one calm and connected. It also never hurts to have a "helper" to maintain milk supply should you need to EP for a while!