You are correct. Leviticus 18, a part of the laws given to the Jews and not to Christians, is the nearest you can find on this particular topic. Leviticus 18:6-17 forbade the following sexual relationships:
- A woman with her mother's husband
- A man with his father's wife
- A man with his sister, half-sister, or step-sister
- A man with his granddaughter or step-granddaughter
- A man with his aunt
- A woman with her uncle
- A man with his daughter-in-law
- A man with his sister-in-law
- A man with his daughter or step-daughter
Notice that nieces, nephews, and first cousins are not forbidden. Nor does Levitucs 18:6 apply: "None of you shall approach any blood relative of his to uncover nakedness; I am the LORD." The relationship to his niece is not one by blood. So even if we were Jews under the Old Law, this relationship would not have been forbidden.
Most children of a widowed parent are uncomfortable with their living parent marrying again. Intellectually they know it is allowed, but emotionally they see a new marriage was lessening the importance of the first marriage. For this reason it is important to emphasize what Paul taught, "For the woman who has a husband is bound by the law to her husband as long as he lives. But if the husband dies, she is released from the law of her husband. So then if, while her husband lives, she marries another man, she will be called an adulteress; but if her husband dies, she is free from that law, so that she is no adulteress, though she has married another man" (Romans 7:2-3). A man who marries again after his wife has died is not dishonoring the memory of his wife. It may be hard to admit at first, but life needs to go on. Marriage does not extend into the live hereafter. "The same day the Sadducees, who say there is no resurrection, came to Him and asked Him, saying: "Teacher, Moses said that if a man dies, having no children, his brother shall marry his wife and raise up offspring for his brother. "Now there were with us seven brothers. The first died after he had married, and having no offspring, left his wife to his brother. Likewise the second also, and the third, even to the seventh. Last of all the woman died also. Therefore, in the resurrection, whose wife of the seven will she be? For they all had her." Jesus answered and said to them, "You are mistaken, not knowing the Scriptures nor the power of God. For in the resurrection they neither marry nor are given in marriage, but are like angels of God in heaven" (Matthew 22:23-30).