This is an odd grey area, I'm not sure its right to judge right or wrong. But I think you think its wrong, and thats what really matters. All I can do is share my reflections from borrowing my Moms things. My only regret about my crossdressing is the sneaking around I did. I snuck because that was my survival instinct. With what I knew and felt at the time, that was the best decision I could make. I feel bad today because I was a little theif and sneak. Mom knew or suspected. (She knows now) and she knew that was my only outlet. I don't beat myself up over it anymore, but there are some choices I wish I had made differently.
Look at your relationship with your sister, do you think she could cope with her little brother wanting to dress like her sister? If yes, then cultivate that relationship and tell her. Having an ally and someone to talk to and maybe shop with will be a lot easier and way more fun. My wife is that partner for me, and its way more fun this way. No secrets, no hiding :-)