Perhaps, but surely the sibling relationship was like before he married her? If not then I may adjust my perception, but if she and her sister were always this way then I am sorry but that's just what it is.
I don't see it as sexual unless the intent is there. In my case not so. Regardless,traipsing through someone's private online history is definitely not acceptable. It's something I am afraid gets my back up. So I apologise if I came across abrupt.