Q: How did Adam and Eve’s family grow or multiply? Did they (the parents) mate with their own siblings/children? And was it not immoral in the sight of God?

A: Adam and Eve had 63 children together before the default, and 42 children after the default, besides Cain and Sansa, who were both children of joint parentage with mortals:

74:6.2 Adamson was the first-born of the violet race of Urantia, being followed by his sister and Eveson, the second son of Adam and Eve. Eve was the mother of five children before the Melchizedeks left—three sons and two daughters. The next two were twins. She bore sixty-three children, thirty-two daughters and thirty-one sons, before the default. When Adam and Eve left the Garden, their family consisted of four generations numbering 1, 647 pure-line descendants. They had forty-two children after leaving the Garden besides the two offspring of joint parentage with the mortal stock of earth. And this does not include the Adamic parentage to the Nodite and evolutionary races.

Adam and Eve did not mate with their own children, but their children did mate with each other, in order to increase the size of the Violet race:

74:6.9 The practice of some subsequent nations of permitting the royal families, supposedly descended from the gods, to marry brother to sister, dates from the traditions of the Adamic offspring—mating, as they must needs, with one another. The marriage ceremonies of the first and second generations of the Garden were always performed by Adam and Eve.

Far from being considered immoral, this was the technique by which Adam and Eve’s family were to multiply, so that by the time they had reached sufficient numbers, they could then begin mating with mortals so as to upstep the races of mankind with an infusion of the Violet genetic heritage.

In our times, we call such unions “incest, ” and it is definitely not considered a moral act, but that is primarily because such unions can produce serious genetic problems, such as hemophilia. In the days of Adam and Eve and the Violet race, there was no other way for Adam and Eve’s family to increase. And, the children of Adam and Eve were a pure race, not contaminated with any biology other than their own.

Please see Home Life of Adam and Eve.