Camille learned her letters very quickly when she was a toddler and accepted each letter as just a letter. Meghan had been working on learning her letters, but the names and sounds just did not seem to stick with her. I thought maybe she was just overwhelmed by the volume (26 is a lot for a little kid), so I decided to scale back and removed all letters in our stash that were not in her name. Very quickly I discovered that she is not opposed to learning all the letters of the alphabet, she would just like to see that each letter has a purpose. An "A" is just an "A" until you make it part of her name. Suddenly "A" is the coolest letter on earth! Just like that, Meg has not only learned all the letters in her name, but she can also spell her name. Now, I have to dream up other words relevant to her so she can learn the rest.
Numbers seems to work the same way for Meghan. She knows 2 because she is 2. She knows 3 because she will soon be 3, and she knows 5 because Camille is 5. She is happy to discard all other numbers because they have no relevance to her.
I have to keep reminding myself that every kid is different, even if they are siblings.