I want to give a heads up that this is probably the less invasive consent breach.
Just stepping aside from the OP's specific story, consider this. My rule of thumb for honest consent derives from something that was used in an Australian promotion on consent and that is "it isn't just a yes, it's a hell yes!!". Enthusiastic participation. The idea is that people can be pressured into agreeing with an "I guess so", or "oh alright" which is not the same as genuine, un-coerced consent.
Now in a case like this, if the OP is being honest and said to himself, If I tell SiL what I'm doing is she going to say "hell yes!" or not? And this is a simple test you can apply anytime you have to consider someone else's consent. What would they say if you just asked them?
If you're not considering that then the answer is pretty obvious, you know full well what they would say.
Unfortunately there is an awful lot of non-consensual porn online, all kinds of stuff that is in most places actually illegal such as upskirt and other invasive photography right through to "sharking" and similar sexual assault activities. Just because you see it online doesn't mean it's ok for you to do it.
On the other hand there are plenty who are into having random guys playing with their underwear, even cumming in it before they wear it. Someone posted something similar here recently,
@subdream mentioned used panties and there are tons of amateur people selling them online.
Bottom line though, if you don't have a "hell yes!!" then you don't have consent.