He asked: O sheikh! whenever I tell my wife about my intention to marry a second wife, she normally threatens me that she will pack her load and leave the house. Please, how will you advice me and her?
O female slave of Allah (the wife)! Fear Allah, for the right of marriage lies with the husband, and it is not for a wife to prevent her husband from taking a second, third or fourth wife.
It is Allah who has ordained this law from above the seven heavens, and He knows what is good for men and women. He has permitted mathna for many benefits, and some of these benefits go to the first wife herself.
The husband could be a miser before, but Allah will make him generous to the first wife after the mathna due to the fact that he must be just to’ the two wives (by spending equally on them). Furthermore, Allah could reward you due to your contentment with the mathna (law of Allah) or even provide the second wife with kids that will be your only source of help in the future. Or maybe your husband dislikes some attitudes in you, and when he takes another wife, he finds out that you are even far better than the new wife, then he may cherish you more.
The most serious part is that, there is a grave danger in you preventing your husband from mathna because Allah can void your deeds, as you have disliked part of what Allah revealed in Quran (Suratul Nisa: 3). And Allah said:
“That is because they disliked what Allah revealed, therefore, He nullified their deeds.”
And negating only a verse of the Quran is sufficient to nullify one’s good deeds.
…May Allah give us a better understanding & guide us to accept the truth