A very thin nightgown or sleep shirt- she can at least show him her nipples through the fabric, or if it's short enough (scissors!) he might see her pussy as well. She can wear a robe over it when/if others are around, but open or remove it when it's just you, her and him. he might even figure out she likes a bit of attention and give her more
Looking forward to hearing about your trip, and hopefully her exhibitionist dislpays for her brother!