Is there a way to pre-prevent or prevent cat fighting

While my aunt's house is being renovated, she will live with us for 2 weeks or more. I have a cat and my aunt's cat is actually my cat's sister. When my cousins brought their cat over, my cat's brother (lol we all got a kitten from that litter) and they were very mean towards each other a fighting . I don't want them to be in the same house and fighting. Can I give them both catnip? lol please give advice

Yes, you can give them catnip. I don't know if that will help as catnip usually makes my cats more hyper and aggressive. 

If you are talking just two weeks it might be better to just confine your aunts cat to the room she is staying in (or a bathroom if she's crashing on the couch). See how it goes after a day or two when everyone settled from the initial move in and keep them separated if there are problems. 

Personally, I don't think I would bring my cat to someones home if they already have a cat; unless they had an isolated area my cat could stay without contact with the other cat. I think the stress from that would be equal to the stress of being boarded and rather than risk injury or long term behavioral problems I would board my cat. But I also know my three cats form a pretty strong pride and have seen them support each other when a strange cat is around. So them going into someone else's territory would end badly for one of the groups, slightly different situation. But in terms of cats relationships that the two cats are sisters doesn't make any difference.

Cats don't form family relationships like we do and your cat may not even remember her sister let alone recognize her. Even if they did, they are now adult female cats with no emotional bond. As far as they are concerned they are competitors for food and resources because they don't understand what is happening. 

That isn't to say they can't get along or wouldn't if given time and space to do so. I just don't think for the time period it is worth the stress of trying to figure out a way for them to get along. If you must, do a slow introduction, make sure they have their own litter boxes and feed them separately at first then move the food closer together until they are eating near each other. Have toys and/or treats scattered around the house to distract them if they start fighting (never physically try to seperate a figh between cats).