All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
It appears from your question that this woman is not your husband's wife and she is a marriageable woman to him. If this is the case, you should advise him and remind him of Allaah and that he should cut this sinful relationship with her. You should advise him with good words and in a soft manner.
If he had married this woman before and she mothered him this girl, then she is his daughter and he is obliged to spend on her [daughter] and give her other rights that he should fulfil towards his children as a father.
If your husband was never married to this woman, then this girl resulted from an illicit sexual relation and thus an illegitimate child who can not be traced back to him and therefore he is neither obliged to spend on her nor nurture her.
As far as you are concerned, you are not required or obliged to do anything towards this woman and her daughter. Unless, she is a Muslim, in which case she has the right of being treated like a Muslim, and if she is a neighbour, she has the rights of a neighbour.
If the matter is as you mentioned in the question that she harms you, then you have the right of repelling her harm in a good and permissible manner. But if you are patient, then there is good in being patient. Allaah Says (what means): {And if you punish [an enemy, O believers], punish with an equivalent of that with which you were harmed. But if you are patient, it is better for those who are patient.}[Quran 16:126]. Concerning the delayed dowry, your husband is obliged to pay any instalments that are due, and it is not permissible for him to delay the payments without a sound excuse except with your consent. Nonetheless, if he is in difficulty, you should allow him time until it becomes easy for him to repay. Allaah Says (what means): {And if someone is in hardship, then [let there be] postponement until [a time of] ease.}[Quran 2:280].
Finally, you should take it easy and be patient and forbearing about the difficulties which you encounter in order to avoid any unpleasant consequences.
Consequently, you should seek the help of Allaah whenever you try to advise and reform others and there is no harm if you seek the help of the righteous and pious people in order to achieve this.
Allaah Knows best.