But DNA testing is the best way to determin a plants relations to others. Altough they may look similar, it means nothin if they are not closly related.
Take the olive warbler
The olive warbler is the sole member of the family Peucedramidae. It was traditionally considerd a member of the Parulidae, or New World Warblers.
New world warbler
However, recent studies of its breeding biology, morphology, and DNA indicate that it is not very closely related to them. Infact, it was so distinct that it did not fit into any known family, and thus one was created for it.