en_tn/num/30/15.md

then he will be responsible for her sin

This means that he will be guilty of her sin instead of her if she does not fulfill her vow. The full meaning of this statement can be made clear. AT: "if she does not fulfill her vow, she will not be guilty of her sin, and he will be guilty instead of her" (See: rc://en/ta/man/translate/figs-explicit)