They’re Murder Brothers! It’s so complicated.

MOSTLY tho, surprisingly, their innate curiosity and respect for each other outweighs all the horrible shit they’ve put each other through. Which has been ALOT. Lestat destroyed Armand’s coven dictatorship by being his usual ray of obnoxious sunshine The New and Improved Parisian Vampire 2.0; Armand did help orchestrate the death of one of Lestat’s fledglings and maybe tortured her, too??? before manipulating another of his fledglings into a 100 yr relationship… there’s more than that, but those are big ones.
They seem to be over it all, they’ve made a kind of peace, but they have that contentious kind of rivalry and competition that brothers have.
AR was doing this Fan Questions for Lestat thing awhile back, and Lestat spoke directly about his feelings towards Armand, and I think “he” put it very well:
“Armand’s thoughts are almost impossible for me to penetrate, and his boyish countenance often reveals nothing of his true calculations and feelings. I love him and I respect him — and I know that he loves me — but I never for a moment imagine I’m entirely safe with him.” [X]
And: “I love Armand deeply. My view of Armand has evolved over time. The less I fear him, the more I love him. And the more I suffer, the more I come to understand Armand’s suffering. I have never doubted Armand’s love for me. We are kith and kin, Armand and me.” [X]
I think you have to consider how Armand feels towards Lestat, too.
“I hate him only because I cannot imagine my soul without him.” – Armand about Lestat, The Vampire Armand, [X]

^Footage of Lestat and Armand, they go from smooches to claws in 0.3 seconds. [X]