Wa `alaykum as-Salamu wa Rahmatullahi wa Barakatuh.
Do you mean to say that your husband is continuing relations with the woman he fornicated and produced the illegitimate child? If that is the case, he is persisting in sin. He is, however, allowed to provide whatever support he can afford to give the child he has fathered. If, however, you are sure that she is trying to break your marriage, then you are justified in insisting that he keeps away from her.
Almighty Allah knows best.
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