Hi jacqui,
If your sil just has numbness then it is probibly what the other girls have sugessted however if her leg is painful, cold ,and or has a blueish color to it ,it could be a bloodclot, my sister had one during her first preg and it was passed of as a pinched nerve at first and when it was finally diagnosed properly it went from her hip to her knee in lenght.
I dont want to scare anyone but it is very serious if she does have one, it maybe worth asking her doc if shr is concerned.