Spike and Dimitri are twins in Meet the Robinsons. Though referred to as "Uncles", it is unknown if they're actually related to anyone within the house.
Spike and Dimitri are incredibly competitive over who will ring their doorbell. Though they aren't seen in full in the film, a Disney Adventures comic shows that they, in fact, are merely squatting inside the planters for reasons unknown. In the Meet the Robinsons video game, they randomly scream when passing by one of their planters, again for reasons unknown.
Spike and Dimitri play a very small role in the movie, appearing only when Lewis and DOR-15 tried to enter the Robinson Mansion by way of the front door. As stated before, they compete for doorbell rings. Dimitri wins both, though Spike insists this is only by a technicality. They also appear briefly when Bud introduces the duo to Lewis. When Wilbur asks Lewis who he has met and what he has learned about his family, Lewis describes Spike and Demitri as twins but does not know who they are related to which Wilbur admits not even he or any of the Robinson‘s truly know.