In Chapter 18, Charlie comes to visit Lyddie. He has been sent on the train by the Phinneys, and he has "good news." Not only have the Phinneys taken him on as an apprentice, they also want to adopt him. This cuts Lyddie because she has worked hard, keeping her family in mind through all her difficult days of toil. Her sole desire has been to reunite her family back on the family farm, and she has been relentless in pursuit of that goal.
But that is not the end of Charlie's news. Mrs. Phinney, he says, "craves a daughter as well." She wants Charlie to bring Rachel back, and she will raise her as her own daughter. Lyddie feels as if she is losing all she has left. She feels unable to let Rachel go, but at the same time, she knows she must let Rachel have this chance for a better life. In fact, it will probably mean saving Rachel's life. She has developed a cough, and if she stays in Lowell, she might die from respiratory problems.
In the morning, Lyddie wakes Rachel and tells her she'll be going to visit Charlie. Rachel "was as excited as if she could really remember him." This is the point at which "Lyddie brushed away a cobweb of envy." It is to Lyddie's great credit that the envy is as tenuous as a cobweb. She has much to be envious of--her younger brother and sister are to be given a life of relative ease when she is the one who has slaved and worried for them for the last years. But what she seems to be envious of here is Rachel's emotional attachment to Charlie. Lyddie has grown to love Rachel deeply, and knowing she is going to lose her, she envies the love that Charlie will receive from their sister. She had previously wondered, "Will she ever know how much I loved her?" Her strong attachment to Rachel causes her to envy the love Rachel is ready to give to Charlie--but Lyddie brushes that envy away because her love is stronger than her envy. She is doing what is best for Rachel, despite the emotional pain it causes her.