Would it hurt my 9 week olds spine, to be sitting her up?

We do a little bit of tummy time, but she never lasts long before she's screaming so she doesn't really like it. What she loves is to be sat up. I just set her on her bum, hold her little hands and she supports herself very well. No probs with her head wobbling either. She doesn't slouch tons, but isn't sitting straight up of course, but my mother said to not do the sitting too much as her spine isn't strong enough to support doing it? Sounds overly cautious to me, as I see first hand how well she's doing with sitting and it's not like I do it for hours. What do you think?