1.
Tibalt, Rakish Instigator's static ability shuts down the standard version of soul sisters.
His -2 allows him to generate a Footlight Fiend, a rather useful creature for blocking. This ability can also be reused with Chandra, Acolyte of Flame on the battlefield.
2.Saheeli, Sublime Artificer's static ability creates a servo upon casting ~87% of the deck. Providing you with the efficient protection, much like Runaway Steam-Kin, just for playing the game.
Unfortunately her -2 doesn't serve a purpose in this deck.
3.Sarkhan the Masterless's static ability works together with his -3 to damage any creatures your opponents attack you with.
Sarkhan's +1 turns all of your planeswalkers into 4/4 flying dragons, that can still use their abilities. A useful move when going for the kill.
Using Sarkhan's -3 you can make a 4/4 flying dragon for blocking or attacking. However in most cases this makes a better blocker, to combat flyers and prevent lethal damage.
4.Chandra, Acolyte of Flame's 0 allows Chandra, Fire Artisan to use her -7 two turns quicker.
Her other 0 makes 1/1 elementals that can be buffed by Chandra, Novice Pyromancer or just used to get in some quick damage.
Lastly her -2 allows you to recast all instants and sorceries in the deck. This is her most useful ability and being able to use it right away makes Chandra, Acolyte of Flame a must have in the deck.
5.Chandra, Fire Artisan's static ability makes targeting her a tough choice for your opponents, it also makes her ultimate ability that much more worth working for.
You can +1 Fire Artisan to exile the top card of your library, and play it this turn, this allows you to get missed land drops, as well as cast another card for Runaway Steam-Kin
Chandra, Fire Artisan's -7 ability allows you to do the same as her +1 however it exiles the top seven cards of your library instead. With a Runaway Steam-Kin on the field this should almost always be victory.
6.Chandra, Novice Pyromancer's +1 temporarily buffs Runaway Steam-Kin.
Her -1 allows you to cast Chandra's Triumph, Lava Coil, and both of your creatures.
Perhaps her most niche ability, her -2 allows for 2 damage to be dealt to any target. This ability may not be her best, but it serves as emergency removal.
7.Chandra, Awakened Inferno can be cast as early as turn 4 using Runaway Steam-Kin. Her being un-counterable, means you get at least one use of her abilities.
Her +2 allows you to give your opponent an emblem that deals one damage to them on their upkeep, putting them on a clock for the rest of the game. This ability is the reason she is in the deck.
Chandra's -3 acts as a board sweep for any non elementals. It kills ~70% of creatures in standard.
Lastly her -X works as targeted removal against both creatures and planeswalkers.
All of these cards serve to bolster the deck in their own unique ways, keeping the deck running at its maximum potential.