Laura - I've never heard of lining up being an sensory thing. I know that lining up and things like stacking, basically the repetitive behaviours, are activities kids use to calm themselves because they're about order and control and predictability. Certainly not something that should have been discounted. Are you able to get a second opinion?

Fiona - First, congrats
My DD also struggled with a newborn in the house. She regressed quite a bit and would do things to try and hurt him. Sometimes I would think she was going in for a kiss and then she'd wrap her teeth around his ear Mike Tyson style. The biggest issue with babies and ASD kids is that babies are unpredictable and make lots of unexplained and loud noise. Social stories help a little bit, and it gets better once the baby gets a little older and can interact. Good luck!