I think the best way to handle this is to put YOURSELF in the young girls place.
I look at this like' well what if it was me?'.
Sure, I'd be scared as anything about telling my Mum. I KNOW she'd be disappointed, but it doesn't mean shes going to stop loving me- or kick me out of the house, or disown me or anything. I look back at my teen years, and I'm sure there were countless times when I made choices that disappointed my parents, but now I'm 28 and they still love me. I'm sure you can all relate to this in some way.
No one is perfect, but that's why Christ died...to redeem an IMPERFECT world of sinners.
The point I'm trying to make is, although she is scared to tell her Mum, she really does need to. I don't know about you, but even if I kept things from my Mother, she always had a way of finding out (some Motherly instinct or something)... then when she DID find out and confront me, I'd be in MORE trouble, than if I'd just been honest and up front with her in the first place.
So I think, you should try and encourage her from a Mother's point of view. I mean, she's not the only one to blame right? Your son was involved too, and I can't speak for you and your feelings about his behaviour, but I'm sure even if you don't approve of him having sex, you still love and support him...right? So then use that as an example, and tell her, 'Hey okay, so you messed up. Your Mum might be upset or even angry for awhile, but she'll get over it, and it won't make her stop loving you. You need her support right now- so you really should tell her'.
At the end of the day though, you cannot force her. I mean even if you tie her up and drag her to her Mother's house, you can't physically make the words come out of her mouth, and I really think if you USE force, she may come to resent you for it later.
So just keep talking with her, encourage her to tell her Mother. Let her know she will always have the support of you and your family, and also her Mum (if you're sure her Mother is going to support and not go mental at her)...then just pray!
I'll be praying, please keep us posted!