I read a line in some random article about childfree somewhere that quoted multiple parents as saying, more or less, "Children bring so much joy into your life! And they do it while forcing you to cut out literally everything else in your life that brings you joy." It really stuck with me.

I'm sure that's more true for some people than others, but it sounds like it has been for LW - it's basically what she's struggling with in this letter, rephrased. She can't respond to her sister because she doesn't have space in her life for any joys other than her kids. LW, if she really is reaching out to you in a way that requires a direct answer, talking about it more like "with raising my kids through this awful year, I haven't had time to find any joys other than parenting, but here are some things I miss from before I was a parent" might help.