Question: Is there a reason why Roger, Anita, and their child are not back for this sequel? Also, how and why did Dipstick get adopted by Chloe?
Answer: We are never told why they are not there and we are not told about how Dipstick came to live with Chloe.
Answer: Because Jeff Daniels and Joely Richardson who portrayed Roger and Anita were busy filming other projects at the time and maybe they chose not to return. Or perhaps the writers couldn't find a way to fit them into the Story. As to how Dipstick got adopted by Chloe, maybe it's just a mystery we'll never know.