>My 34 year old sister has been single the majority of her adult life and for the past five years has been very firm on not wanting children.

If she only started the whole "I don't want children" thing when she was 29 and was single most of her life, it does sound like a bit of a cope. "It's ok that I'm pushing 30, not married, and never had a real relationship, I never wanted kids anyways!"