How to reproduce:

1. Install Sit and Create
2. Make a simple train with atleast one stair or slab on it
3. Assemble the train
4. Try to sit on the stair or slab and notice it doesn´t work

Sit works on fixed block positions, and the entity that is spawned and the player rides on to achieve the sitting visual is not moved ever. So this probably needs some finagling with Create directly.

Alright, I reported it on their side:
Fabricators-of-Create/Create#1522

I meant it needs finagling from me to make it work specifically for Create.