For example, when my brothers were still little, as I was quite mature, my parents told me when the middle one had a tumor.
However I do realize that in the last years, since I live far from home, my brothers probably feel lonelier without me and I've not been as present for them as I should have been. They have had their hardships too and I wasn't there to help. That makes me feel really guilty.
In the end I don't think the fact of being the oldest/middle/youngest brother has much to do with feeling lonely, as the OP stated.