It is likely that the couple listed just after Rebecca and Isaac (Thomas Sisom and Rachel Brelsford/Belford) in "Marriages Authorized" were Rebecca's parents. See IsaacSiddallDocs link under Isaac. The older couple married in late 1729, and Rachel would have been rather old but probably still within childbearing age when Rebecca was born. Rebecca's father was Thomas Sysom, and I have not found a record of a second marriage for him if one exists.
ChildrenNote 1
I found John, James, and Isaac Siddall listed on Columbiana County tax records for the year 1818. John was the original proprietor for one of his plots while Isaac was the original proprietor for the plot of James and one of those of John. There is no listing for William, and the father Isaac was deceased. Since he had died very recently, his name may not have yet been been removed from the records. I haven't followed them further except for confirming through the 1820 census that Hanna Burk and Thomas Sisom Siddall were in Columbiana County at that time and that Sisom Siddal died in Indiana in 1863 (findagrave.com). Sisom was probably Isaac's and Rebecca's grandson.
Marriage to Rebecca Sysom: see IsaacSiddallDocs.html (link below).
Isaac made a will in 1817, and it was submitted for probate late that year. In the will, Isaac mentions wife Rebecca, sons James, John, and William, and 'only daughter Hannah, relict to Samuel Burk.'
Supporting documents for Isaac, Rebecca, and their family.
Note 1: Isaac, Rebecca, and son Thomas are mentioned in Quaker meeting minutes of 25d 8m 1770 in Fairfax, VA, Meeting presenting certificate from Middletown, PA, Meeting. They were accepted.