It's just me and my "little" sis, although the distinction seems faintly silly now that we are both in our mid-40s. We were born 18 months apart, after our parents had struggled to have children for the first seven years of their marriage. They have now been married for 56 years, still bug the hell out of each other, and still couldn't live without each other.
As they began to think they couldn't have children, they began seriously considering adoption, and around that time they had me and my sister in fairly rapid succession. When it rains, it pours, I guess.
I'm not sure how the adoption route would have gone for them; we are members of a religious minority, and in those days cross-religious adoptions were nearly unheard of.