Is it permissible to boycott the husband's family who are seeking to spoil the marital life, given the husband's desire to reconcile with them and the presence of a female child?

You are not obligated to visit your in-laws or maintain ties with them if they are not your blood relatives, especially if you have been harmed by interacting with them. However, it is not permissible to abandon them. Your husband does not have the right to force you to visit them in this situation. Nevertheless, a wife's kindness towards her in-laws and her overlooking their mistakes is part of perfect kindness towards her husband and noble character. So, if you can maintain ties with them without harm, that is preferable and better.