My interpretation of Diamond Tiara and Silver Spoon's behavior is that they have antagonistic, but not abusive, interaction with the CMC.
Like if the CMC made a go-kart, SS and DT would probably say something like "hey, blank-flanks, pretty dorky Go-Kart", but wouldn't try to sabotage it or break it. Then the CMC would spout their comebacks and challenge DT and SS to make a better go-kart, wherewith such would happen, and they would race.
Sure, DT and SS tease the CMC, but they get as good as they give, and it's not truly cruel. It's more of a rivalry than a picking-on sort of thing.
I don't think that DT and SS have any friends other than each other, especially given that they interact with the CMC, who they don't like, more than they do other non-blank-flank ponies.
Maybe, other than each other, the CMC are the closest thing that Diamond Tiara and Silver Spoon have to friends.
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