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.
First of all, you should know that marriage in Islam has some specific conditions the most important of which are the consent of the guardian and the presence of two witnesses. Therefore, if your husband married this woman with the consent of her guardian and the presence of two witnesses, then she is Islamically his wife and there is no need to invalidate this marriage.
The fostering of the child is the right of both parents as long as the marriage is still ongoing, and if divorce takes place, then the mother has more right in fostering the child as long as she does not remarry. If the child is a son, then his fostering continues until he reaches seven years old, then he is given the right to choose between his parents. If the child is a daughter, then her fostering continues until she reaches seven and then her fostering moves to her father if he has a female relative with him who is suitable to foster her.
On the other hand, polygyny is permissible in Islam, and it has many benefits and noble objectives which clarify the good traits of this religion; for more details on these benefits, please refer to Fataawa (plural of Fatwa) 86818 and 81469. However, among the conditions of polygyny, is that the husband should be able to afford the obligatory spending of both wives and be just (in staying overnight) with them.
In any case, it is not a condition that the first wife has a deficiency and the like so that the husband can marry a second wife. Hence, your husband was not unjust to you by marrying a second wife, and his family did not wrong you if they had advised him to marry a second wife especially that he needed to marry a second wife to have children, as this is a natural instinct for which a person is not blamed.
Besides, your husband is not obligated to divorce you, and if the law prevents him from having two wives, then the law should not be taken into account if it contradicts the Islamic Law. So, if he is obliged to divorce you, he may do so on papers without uttering the word of divorce, because when writing divorce, it does not take place without intention.
Finally, we advise you to embrace Islam as it is the religion of the truth by which Allaah abrogated all the previous religions and you may read an introduction to Islam in the following Fataawa 88051, 88906, 81979. Also, there are many false beliefs and contradictions in the Gospels which you should open-mindedly investigate; for more benefit, please refer to Fatwa 85307.
Allaah Knows best.