Tell you sister and she if she can digg deeper into the situation.

You can feel it actually

idk

I personally think you should tell your sister, and let her deal with it.

It seems as if you already know the answer to this question. However, by the accusations you've stated it does infact seem he is cheating on your sister. Try to think outside of the box, maybe he's trying to plan a surprise vacation or arrange some sort of party..?

another way to tell is if he accuses her of cheating. a person who accuses there loved one of it 9 times out of 10 has thought about cheating or has cheated. they know that if they can cross that line than there loved one could too.