There is no fucking way any husband would do somerhing like this to that extent unless approached by his wife, wife and sister, the sister, or perhaps sister's mom and dad for a professional shoot. A few candid photos of her casually around the house for the family photo album is one thing but this much effort isnt something a husband would do in secret. It's just weirdly inappropriate. Perhaps sister approached him directly to get some tasteful photos for her online dating profile and you have some unreasonable distain for online dating and they didnt want to upset you.. Outside of that it would be inappropriate since that seems like something sisters would talk about and she didnt mention it either?. Its all very weird no matter how you look at it.

I dont want to say he is grooming your sister - but he's grooming your sister. At least making some sort of low level effort. She gonna be 18 soon so watch his behavior carefully.

Also, dont listen to me or anyone else. None of us know what we are talking about. Trust your gut. Keep investigating until you are satisfied.