Spending a lot of Time with the SISTER?

Are you serious? Of course it's inappropriate to spend months with another woman, especially when you will be together with her 24/7 in places where no one would ever know for sure if this 'friendship' turned into a month of cheating. Find a guy friend to go with. Put yourself in her place would you want her to go off hiking with a guy for months. How much more temptation could you possibly put yourself in?