The premise of this third episode is Dan installing a stair lift chair system for Roseanne. The second episode of the series clearly shows the chair already installed.
Harris was born in the last season of the original run of Roseanne, and here she is said to be 15 (Darlene says she has to put up with everything for 3 more years when she turns 18). She would technically be 20 if this took place in 2017.