Sentence:  Henrietta , however , was at that time engaged in a lengthy correspondence with Joe 's older and more <WSD>serious</WSD> brother , Morris , who was just about her own age and whom she had got to know well during trips to Philadelphia with Papa , when he substituted for Rabbi Jastrow at Rodeph Shalom Temple there during its Rabbi 's absence in Europe . 
Meaning of the word marked with <WSD> tags:  concerned with work or important matters rather than play or trivialities