If you wanted to make your and your sister's deck faster, more efficient, and more powerful, you should look through your cards to find strong cards that you have more than two of and then build a deck around the card or its type. Rapidash is the closest Pokemon you have to those guidelines at the moment. From a competitive perspective, the better Rapidash that you own is the one with 90 HP with the Burn attack for one energy and the 40 damage attack for two. It is a very fast card, and when combined with Full Flame, its first attack can become a big pain. If you combine that Rapidash with trainers that heal it or return it to your hand, you can make a very focused, fast, and strong deck.
You could also try getting more copies of any of the other cards in your deck. The Victini with V-Create can be very fun to play if you play a deck that fills up its bench very quickly with cards like Pokemon Collector, Dual Ball, Quick Ball, Level Ball, and other trainer or supporter cards that get a lot of basic Pokemon out quickly. The Audino works well with the Victini that lets you reflip coins because it allows you to statistically increase the damage of its first attack. Any cards that attack extra energy to your Pokemon would work well with the Audino. The Druddigon is fun to play with healing cards while having a Rocky Helmet or Eviolite attached to it. Lastly, the Magcargo is great in a deck that uses four of itself, four Slugma from Undaunted, no other Pokemon, around 20 Fire energy, and Stark Mountain. Some of those decks might be hard for you to build because of the stretch you have to make to get the cards, but since I don't know your collection, these are all ideas you can try out if you have the right cards.
When it comes to deck-building, having a focus is key. Knowing what you are trying to build makes your deck all the more effective when you try to play it because it lets your deck work well during each and every game and leaves less of figuring out what you should do during your turns to guess work. I hope this is the kind of advice you are looking for, and have fun playing!