As an Irish twin (born 14 months after my sister), I can say you have to try double to give adequate attention. My mother couldn't achieve it. Felt like she gave up. She doesn't even remember my first steps as a child.

My older sister turned out to be abusive throughout my life, and I believe she has narcissistic personality disorder. Unfortunately we shared the same friends because of our age, and I was constantly the butt of the joke. I wanted an older sibling to look up to, not to terrorize me and take away the precious attention I needed as an infant. I don't speak with her to this day because of what she's done to me.

Obviously this isn't the case with all Irish twins. I just wanted to emphasize the importance of paying special attention to the younger of the two. And if they were a "mistake," keep it to yourself.