What matters is this: whoever she was, Aunt Esther Gubbins was right. (Para. 19)
Meaning: What is important is that no matter who she was, Aunt Esther Gubbins was right.
matter: vi. be important
In a composition, teachers are concerned that students express themselves correctly, but what also matters is that the thoughts are complete and well sequenced. 在批改作文时，老师会注意学生表达是否正确，可同样重要的还有表达的思想是否完整以及组织是否合理。
The cost of heating a home does not matter to someone with a lot of money, but for families with low incomes, it is a matter of great importance. 对于有钱人来说，家庭取暖的费用算不了什么，但对于低收入的家庭来说，却是一件大事。
whoever: 1) no matter who
Whoever broke the window, he should pay for the damage. 无论谁打破了窗户，都得赔偿。
2) any person that; who
Whoever wants to go may sign their names on the paper. 凡是想去的都可在这纸上签名。