Hello del, The
Indiana Holiday World and Splashin Safari which is located in Santa Claus, Indiana is available from early May to early October, and your sister has chosen the perfect time. It's not overcrowded because the fireworks has been finished on July 4th and most of them are still in holidays here. From June 28th to August 2nd is the exceptional busy time, your sister will be there in late August, so not a problem at all.
Both Holiday World and Splashin Safari give everyone a perfect family vacation. Some of the highlights of the Holiday World are the theme park, wooden coasters, The Voyage roller coast and kiddie play zone.
Some of the highlights of Splashin' Safari are Jungle racer five-story water slides, family water rides, Bahari giant themed wave pool and water Rides for Little Ones.
Only I have listed out few, if she goes there, she'll find many, convey my wishes to your sister & her kids!