My 12-year old friend wanted to create a account on gmail on my computer and now she can't sign up at all because she's under aged but my 14-year old sister wants to create an account, how can she disable the thing to let her sign up?

You ask your parents if it's okay for you to have a gmail account. If it is, then you lie about your age. I'm shocked you didn't think of that.

?? I never even said my age, how do you know what my age is anyway?

I don't know your age, but I do know your sisters age, you wrote it in your question. I was answering the question as if I were speaking to you, but it was meant for your sister. But, seeing as your sister would have the same parents, the advice does apply to you both.

Oh sorry and thanks