The Sadducees are quoting what Moses had written in the law. Moses' quote can be expressed as an indirect quote. Alternate translation: "Moses wrote for us that if a man's brother dies" (See: [[rc://en/ta/man/translate/figs-quotations]])
"have a son for his brother." The man's first son would be considered to be the dead brother's son, and the son's descendants would be considered to be the dead brother's descendants. This can be stated clearly. Alternate translation: "have a son who will be considered to be the dead brother's son" (See: [[rc://en/ta/man/translate/figs-explicit]])