It is a tricky one, with long distances, as it's difficult to judge where people are in their grief if you don't see them regularly.
What do you do, keep sending messages which you don't know how they'll be received- are you reminding them of their loss when they've had a good day for once and maybe reached that point when they feel guilty about those moments when they "forget"? Are you, associated with their life from "before", when your brother was alive, just a reminder of their loss?
What do you say on a text, or even a phone call? If you can't physically be there, you can't help, or do anything.
Is the contact the same as before? Are they ignoring you and not responding?
There's a recent grief thread which remarks on the fact that many people seem to think the done thing is "getting back to normal", back to school, work, almost as if anything else is just prolonging the pain. Maybe they're sticking to the same relationship you had before, and the same level of contact, in that belief. Some people don't want to have to do the "I'm fine" thing when asked how they're doing, and would rather not be asked...
If you want to contact them, do. If it were me I'd be pleased that you could do that, and know I'm doing what you need, not what I think is expected, iyswim...