I ... actually think that's a rather tactful and nice letter. Could be slightly better but not bad at all, given that it's raising a difficult issue. She's also given you time to take it in and think about your response. It might have been a conversation that's easier face to face, but I can also see the point of giving you time to take it in and think.
I can see how it's a shock though.
You are very upset by it. May I ask why? Ie, exactly what it is that upsets you?
Is it that you thought you were very close already?
Is it that you hadn't realised your mother felt this way?
Does it feel like your mother is saying that you -aren't- close?
Is it just a shock that she'd say something like this at all.
If you feel you are close to them, then it sounds sufficient to say that you do feel exactly that - that you are close to them. That perhaps they would like even more contact but that you feel that you do love and are close to them, it just doesn't need or feel right to have to ring them/brother every few days. (and once a week seeing them sounds really quite enough to me).
Maybe when you speak with her, say thatthat contact between parents/brother can't healthily be forced, but you do love them and feel very happy with them and that when, eventually, your mother is no longer here you really anticipate that you and your brother will be fine.
Also that you don't feel that it is -necessary- to automatically see them just because you're in the same town, when you see them every week as it is!
Is it possible to take a level headed and reassuring tone with her, while standing your ground?
Then I also think that after you've had that conversation with her, then in a few days it might be worth ringing your brother and asking him outright if he's happy with the level of contact. ATM you are happy with it, but unless you've discussed it you simply don't know if he is. You can't necessarily assume, now that the question's been raised. It's worth asking. Then once you know the answer, you can decide to keep things as they are, or make an effort to see him more often. Or less even :p
I can understand how it's a shock, but your assumptions have been shaken up here and it's an opportunity to consider for yourself exactly how much contact you are comfy with - and reassure your mother that you won't drift apart.