My brother in law recently asked me to find out my sisters ring size. I already call him my brother in law because they have 2 kids together. Idk how to ask her without her getting suspicious. Any ideas?

My bf told me to tell her that he asked me for my ring size and say idk mine and casually ask her if she knows hers. She's gonna ask a lot of questions about him asking me so idk if that's a good way lol although it will distract her from me trying to figure out hers.