This is another example of the many times "Little House on the Prairie" was guilty of "jumping the shark". It is a common tactic of long-running television programs to add children when the original ones get too old to be cute.
The Ingalls family added three children (by the time they leave Walnut Grove, three of the five children going with them are not biological) themselves. While adoption was not unheard of in those days, the constant adding of children on this show by adoption bordered on the ludicrous.
There is a tender moment when Harriet Oleson speaks to Nancy through the door at the orphanage and tries to convince her to come home with her. We find that Harriet herself was raised as a spoiled child and acknowledges that she did the same with Nellie. Her statement that she will love Nancy exactly the way she is is moving, even though it's misguided.
Props to the scene where Willie chokes on his sandwich when introduced to his new sister, "This is my surprise? You finally got Nellie married and out of the house and you bring home another one!"