Cats need, wherever possible, to remain within their own territory. The ideal solution is to leave your cat in his/her own home, and get someone to come in to feed him/her.
Second best (but still far less good) is to use a cattery.
If you leave your cat with your sister, she'll have to ensure that he/she remains inside at all times, otherwise the cat will almost certainly try to make its way home.
Chris
PS: If you can persuade someone to look after your cat, please ensure that they're in good health. I went way on Christmas Eve, leaving my cat in the care of my neighbour. When I got home at the New Yea,r I found that the cat had never been fed at all (and was, understandably going berserk for food when I walked in). I later found out that my neighbour had been found dead on Christmas Day.