It's hard to know if there is truth in that old wive's tale or not. I can't really compare, since my mother had a c-section with all 8 of us. Her mother delivered 7 naturally (haven't heard the story - should ask sometime!) Her one sister has 6 by c-section, and the other has two natural. (Labour under 10 hours, I think.)
My sister has two so far, with about 14 hours of hard labour, for each.
I have two so far, with about 7 hours of labour, each. (I have a much easier time than she does.)
I think they are figuring that you might be built like your mother, and so have a labour like hers. I am not built like my mother, but my sister is. (Which makes me wonder if c-section was necessary for my mom. That was 1980.) Anyways, I don't know if that helps you at all. Hope all goes well for you!