Maybe position her as if you're doing a photo shoot, using a partially concealed scale, then have another camera set up surreptitiously to record the final tally. It might even be constructive for her, sort of like an intervention ("You can't deny it anymore, sis, you're 50# bigger than your older brother - time for some diet soda.")
As for he spy shots, it's only wrong if Tadao says it is.
