Should I always pick-up my baby when she cries?

with our 1st, we picked her up when she cried. I had read it gave a sense of comfort. We also allowed her to sleep with us most of the time. Either all night, or in the middle of the night. Now the 1st is turning 6, sleeps in her own bed & is relativly good mannered. Our 2nd is going along the same path. I find that I am sometimes busy with the 1st & cannot pick-up the 2nd child when she is screaming. Today, I had to help the 1st with her hair (she has a scalp condition) & the baby (6mths old) had to lie there crying for approx. 5 mins. My 6 yr old asked why she had to hear her sister cry. I told her that I had my hands busy & that the baby would have to learn to comfort herself BUT I ended up picking her up & putting her on my lap. I had to do the 6yr old's hair with 1 hand (kind hard to do). I was a little upset & vocal. I want my older daughter to be good with the baby but I find myself mumbleing unkind words sometimes, & I think my older 1 hears. How do I keep my mouth in check?