I want to know what the problem is? I was a mechanic for years and I own over $10,000 worth of tools that I almost never use but they are mine and when I need them I know where they are, why do I mention this? because I married a woman that can't fix or repair anything so she keeps her hands off my tools and that makes me happy.
I say you should tell your wife that you love her work, and really wish that if she ever has any questions or wants to know how to do something she should ask you and you will be more than willing to help her in any way you can if she wants your help, then reassure her that as long as she enjoys taking, looking at and showing off her photos your happy.
As for your Sister in Law, as long as she isn't buying the food for your house let her know that your opinion of how to compose a photo is far less important than that of her clients, they are paying her, you would only be offering your artistic opinion of what you think of the photo if your not paying her for her photos they don't have to appeal to you. If you like her you can offer to help her out with things she is having trouble capturing but you would prefer that she be specific about what help she needs, and that she should not ask for artistic advice beyond what technical things she whats to know how to do to produce a desired result, for composition and posing subjects she is on her own.
Then let the both of them know that you would prefer that they not ask to borrow your equipment without at least having you present for the photo shoot, if you let them use the equipment at all.
You have not lied, you made your point, you offered help, you offered support, you offered encouragement but most important you have saved your equipment from their hands.
There is an old saying "Lend out your wife, she can take care of herself, don't lend out your tools" I would like to add "Don't lend your tools to your wife or neither may come back undamaged".