Is there a male and female milo tree? Or are there cousins of the milo tree that look simalar?
As far as I can ascertain, Thespesia populnea (milo tree) is not dioecious, meaning the trees are not divided between male and female. The Thespesia genus contains 18 species (T. cubensis, T lampas and so on) which would fit the description 'cousin'. Whether any of them look similar would require checking a complete list and looking for images - you presumably have a plant of a specific yet similar appearance in mind. Since you can see it and we can't, that's probably the only way for you to check. Alternatively, post images and see if anyone here recognises your tree.
As Bamboo told you, the Milo trees are not separated into male and female. According to this:
Thespesia populnea is monoecious (each flower of each individual has both male and female structures).
Have a look at Wildlife of Hawaii for more information.
As for a similar plant, the same site suggests Hau (Hibiscus tiliaceus).
The similar Hau (Hibiscus tiliaceus) also grows along the coast and has color-changing flowers and heart-shaped leaves, but it has more widely opened flowers with conspicuous lanceolate sepals.
In addition to that site, good information about the Hibiscus tiliaceus, along with pictures, can be found here. It too is monoecious.
Both of them belong to the much larger Malvaceae family but are different in genus, so I don't know if that means you can call them cousins, but they might look close enough for you to enjoy them both.