After thinking about it some I had another thought. You could ask for ask for any gender neutral items and some favorite girl items and explain that even though you aren't sure of the gender you are expecting the clothes back if its a girl. Ask if it would be easier to box them up send them all before she's busy with a newborn or if she wants to take the chance of waiting just be sure she understands you'll need them asap after birth but hopefully you'll have gotten those requested outfits to prepare for the meantime. If you are more worried about causing waves hopefully if would be a gentle enough approach without spilling the beans. I do often find myself in small predicaments when I loan stuff out but I try to remember how it makes me feel when I'm on the receiving end to battle out the frustration. Again good luck!
Andrea -SAHM to Sean 10-09, Owen 1-13, and married to my best friend!