Isabella Stewart of Bonkill, Countess of Mar

Correction, to clear up the confusion: This Isabella is the daughter of Alexander Stewart of Bonkyll and Jean fitz Alexander (aka Jean Fitz James), daughter of Alexander Fitz James. Isabella married Domhnall II, Earl of Mar and with him had a son, Thomas of Mar; and a daughter, Margaret of Mar (Countess of Mar).

Isabel, daughter of John STEWART of Bonkyll, by Margaret, daughter and heir of Sir Alexander DE BONKYLL. She was living 16 July 1351. [Complete Peerage]

Jim Weber notes: "I diverge from CP on this. While not a resounding endorsement of my line, Burke's does lend it the conceivability that the Isabel cited by CP married Donald, Earl of Mar; while her niece, another Isabel married Thomas Randolph, Earl of Moray. They could not be the same Isabel because their husbands both died in 1332, less than a month apart."

Stirnet notes her to possibly be daughter of Alexander Stewart of Bonkyl (son of John Stewart of Bonkyl).

Can anybody provide proof what scenario of parents is correct?