R.T.
this may not be the best solution for you, but why not try and make up something. like you saw something and then get your 2 year old to go with you so that she can see what it was and that it isn't anything to be afraid of. do this a couple times and maybe she'll start to understand that everything she sees isn't going to get her. she'll be more inclined to find out what it was than be afraid of it. and of course something you've probable already taken care of, but i'd have a talk with the 7 year old and explain to her that it isn't nice to scare her sister. hope this might help.