Actually it was a vision of the future like the ones Sara & Mao Nome had in Macross Zero. She saw Ozma being killed by the Vajra in her nightmares. That's why she didn't want him to fly anymore. I guess Ozma is going to die later & Alto is going to inherit his VF-25 (another Macross tribute).
My best bet is that Ranka & Sheryl are natural sisters & Ozma is Ranka's adoptive brother. Maybe they are both descendants to Mao Nome... (Sheryl knows French, & Mayans spoke that language. Sara's chant in Macross Zero had French in it).
Btw... The captain at S.M.S. looked a lot like a scar-faced version of Gloval. An older Hikaru Ichijo perhaps? The sole survivor of the Megaroad?