Not only did Spike exploit a sibling rivalry in a sibling rivalry, but he created it in the first place. Spike asked Thorax to impersonate Celestia while she was on important business and instructed him to deliberately brush off all of Luna's security advice to make her susceptible to allying with him for the betrayal.
Spike also hired Grogar to abduct all castle guards and replaced them with changelings so that they would be unrealistically easy to distract.
Shining Armor was also a changeling, that's why he seemed totally fine with his security being circumvented. The real one would of course eventually come for a visit, but then Spike would already hold the crown, and legitimately, as proven by his shrewd scheming.
Grogar never returned the guards. He didn't call back. All guards are changelings now, and they like having a job.
Spike could only convince Grogar to help him by promising him that he could keep the guards and do with them whatever he wants.
Twilight's parents gave him a gold start for this. (According to rumors not entirely on their own volition.)
Pinkie Pie is still inviting Spike to all her parties, and he is the guest of honor. Afterwards there's always plenty of cake left.
What Spike does not realize just yet is that Luna was a changeling all along. A royal one, in fact. And she's after the crown. For that sweet sibling love.