So, is there a context for this? Because if there isn't, I'd like to introduce one.
So, Sweetie Belle found out she's a robot. But her love for her sister had ultimately given her a soul (it's magic, I ain't gotta explain shit), so rather than go on a murderous rampage, she goes into a deep depression. Rarity finds out, but upon soul scanning her to make sure she isn't joking (wasting the Robot Police's time is a serious offense), realizes that Sweetie Belle does have a soul. Since this is her sister's darkest hour, Rarity decides she needs to be there for her, cult be damned.
After a whole lot of bonding, Rarity has been able to move on from the war. The terrible void in her that was once filled by Fluttershy's cult has been filled by the only family she has left. Rarity tells off Fluttershy, vowing to try and stop the end of days.
On the day of the Eclipse, Lord Smooze is summoned. All of Equestria is doing everything in its power to stop him. Rarity and Sweetie Belle are guarding Pinkie while she's gypsy magic-ing up some way to vanquish the Smooze.
It's a hard fought battle, and Rarity is gravely wounded. Sweetie Belle is mad as hell, and, no longer caring if anyone else knows she's a robot if it means avenging Rarity, starts powering up...