At least at the start of the series, Sandy is othered for being a land creature unable to breathe underwater. However, when you think about it, Sandy is vastly more closely related to 90% of Bikini Bottom's citizens (fish) and to Pearl than to literally anyone else.

• Sandy Cheeks and Pearl Krabs are both mammals: class Mammalia, subphylum Vertebrata, phylum Chordata.
• Fish are... honestly a lot of things – highly nonspecific. So we'll just genericize it and say they're also all under subphylum Vertebrata.
• Patrick Star (sea star) is more closely related to the sea cucumber Kevin that he and SpongeBob idolize: both phylum Echinodermata. He's the second-closest to the fish etc., as he's also under superphylum Deuterostomia.
• Mr. Krabs and Larry the Lobster are expectedly related (suborder Pleocyemata, order Decapoda), but they're also related to Plankton (copepod) under the superclass Multicrustacea. If you go far enough up the tree, these three are all more closely related to the nematodes than to the other cast (superphylum Ecdysozoa).
• Squidward (cephalopod) and Gary (gastropod) are similarly far out, being molluscs (along with bivalves, e.g. the ones used as a stand-in for birds). If you go far enough up the tree, they're more closely related to annelids like the (fake) Alaskan Bull Worm than any of the other cast.
• SpongeBob is a sponge (phylum Porifera). He's the furthest one out by far. Everyone else is in the animal subkingdom Eumetazoa, but not SpongeBob. The jellyfish (phylum Cnidaria) are closer to everyone else than SpongeBob is.

Here's a really janky representation of how insanely far apart they are with a very high-level tree of life: