Are you still having trouble playing multiplayer on the mobile version with your sister? (based on this post: https://forums.terraria.org/index.php?threads/one-terraria-an-introduction-future-impacts.61935/page-50#post-1549225). Whether it's the in-app purchased game, or the regular paid-up-front game I would think that they could play together as long as they are on the same network and same multiplayer version.