Maelstrom
Excelllent.  Continuing story of the twins and the planet Petyabee.  I read this a few years ago.  I read all her books and series.  This planet controls what can and cannot be done on it.  If it doesn't want you there you'd best leave.  Best to read the series from the beginning.
Fiction, Brothers and sisters, Science fiction, Twins, Life on other planets
Elizabeth Ann Scarborough, Anne McCaffrey