Twins?

I have two friends who insist they are brothers. The thing is they look nothing alike. So the other day I was sitting in the bus, and I began wondering how I might disprove their claim. The first thought which came to mind was asking them what year they were born in. If they both said they were both born in 1987, it would mean they were twins, and that would be too blatant a lie.

Then I realized that, that wasn't all true. Being born in the same year and being siblings didn’t necessarily mean they were twins. For e.g.: Janine could have been born on the 1st on January, and 9 months later her baby brother Christopher. So actually siblings could be born in the same year without being twins.