My Father also died when I was 7 and I'm now in my 30's. I kept in contact with my Nan until she died a few years ago, but have only seen my cousins and Aunties and Uncles a handful of times since, but I guess it didn't help that they lived quite a long way away.
I can understand you being curious so perhaps ask your Mum why she says that they are odd. Is she angry that they didn't make the effort to stay in touch or is there more to it?
You could write to a cousin or Auntie that you were close to at one time, and suggest you meet up as you're curious to find out more about your Dad's childhood or their family history, and take it from there?