Lower Sanitary T in Wall for new lower sink
You might need to use a Fernco coupling to get everything hooked back up. It's a rubber coupling with hose clamps to hold the rubber in place like this:
Source: SupplyHouse.com No endorsement of the vendor intended or implied - they just had a nice pic.
I'd use it on the 1 1/2&quot; pipe up to the 2nd floor. Slide it on the 1 1/2&quot; pipe, up and out of the way, hold it up with a piece of tape for now.
Glue a piece of 1 1/2&quot; pipe in the top of the new sanitary tee.
Slip the new sanitary tee in place, glue it to the drain going down. The 1 1/2&quot; piece sticking out of the top should just about touch the 1 1/2&quot; piece coming down from upstairs.
Pull the tape and slide the Fernco down over the stub of 1 1/2&quot; pip out the top of the sanitary tee. Tighten the hose clamps to spec.
Use 1 1/2&quot; pipe to finish the trap to the new sink. You could use a screw together (slip joint) trap to make it easier to assemble, adjust, and take apart when someone drops something important down the drain.