Sir Isaac's father, Isaac Newton married Hannah Ayscough, got her 'in a family way' and then promptly died six months after the marriage and three months before Sir Isaac was born. So Sir Isaac was an only child of that marriage.
His mother, Hannah then married Barnabas Smith and had two daughters (Mary Smith and Hannah Smith)and one son (Benjamin Smith). These would be half sisters and a half brother to Sir Isaac.
Soooo, Sir Isaac had no sister by the name of Rachel.