I suppose they do in some cases ~ of course there are siblings who will drift apart and never regain contact because they just don't like each other ;o)
Generally, I think that people tend to spend so much of their time raising children and doing their 'immediate family' stuff that they are too busy for adult socialising. In my parents case, they are enjoying the freedom of being able to take holidays with their siblings, eating out together etc without the pressure of having to watch what their kids are up to!
My sister is 10 years younger than me & we have never been that close, however now that we both have children of around the same age we have far more to talk about. I don't think we will socialise that much once our children have grown up, as we may find that once again we aren't that close!