Hugs! It can be very hurtful when a friend doesn't come through for you how you'd like them to, especially if you've done it for them in the past. And that's on top of the original stress you're dealing with. I think, with a good friend, it can be a good idea to just outright ask for what you want, as others have said.
It may not really be her thing to send something every week to your brother, and even if she's thought of it she might feel like she was just copying what you did. But you could say to her, "Hey, my brother would really appreciate hearing from people, just a card now and then." Or as Eden says, asking her to check in with you every week, or asking her to get together with you for a chat, and so forth.
Some people just really honestly aren't good at thinking about what to do on their own, even though it may seem obvious to other people. If they really care they are usually open to direct suggestions, though.