Chapter 19: Amy, while she stays with Aunt March, learns a bit more patience and humility. She struggles to meet the demands placed on her with grace, but Esther (the french maid) helps. Being away from her family is also a trial to her, but she does her best and it turns out to be pretty good. If I remember correctly, the drawings that she makes of that painting are to be important later.
Chapter 20: Mrs. March does a lot of mothering in this chapter! First, she goes to Aunt March's place and visits Amy, who is still waiting for the danger of the scarlet fever to pass. Amy is thrilled, and shows her mother the little meditation corner that she has built for herself, as well has her copy of Mary & Baby Jesus. Mrs. March, of course, is pleased with her work. Mrs. March also takes over the nursing of Beth, and in addition she comforts Jo who is very upset about Mr. Brooke's intentions toward her sister.
Chapter 21: Laurie, upset because he has not been included in a secret and acting like a spoiled fool, plays a terrible prank on Meg. Once that is all sorted out, Marme swears him to secrecy over the matter to protect Meg's feelings. This causes problems for Laurie at home because his Grandfather hears of the problems and tries to force Laurie to say what's been going on. Jo helps to sooth things over.
The family (and their neighbor) are beginning to grow up and with growth come some inevitable growing pains. Like Kate I think that Laurie is still somewhat lacking. I don't blame Mrs. March for not being excited about the idea of one of her girls marrying him, although he has made a good companion for them. It is, indeed, bittersweet to see the girls growing up and to begin to anticipate the break up of their merry bunch. Marriages change so much. However, a good marriage and happy family will bring a great deal of happiness.