Your wife's objections are justified and completely understandable.
Even if the friendship between your brothers and sisters is deep, I think such a thing is inappropriate.
Confinement is an extremely troublesome and secretive thing, and such a thing can only be done in one's own home, and it is inappropriate to go to anyone else's home, even if it is the brother's family.
During confinement, there are many intimate situations, and pregnant women may also have physical, physiological, psychological and other unpredictable conditions and problems. These situations and problems can only be handled by their own family members, and there is no way for brothers and sisters-in-law to intervene and deal with such things.
Not to mention that during confinement, in addition to the mother should be well taken care of, there are also babies who should also be well managed.
There are many things to pay attention to in the management of the mother's baby, there are also many details to pay special attention to, plus the crying and noise of the baby, can your family bear it?
This requires a person who understands the professional nursing knowledge in this area, and carries out specialized management and nursing.
Your sister is confinement at your house, do you still have to bring such a person who manages and cares for the confinement?
If it were your brother-in-law to serve the confinement, would he serve? Isn't it convenient to wait for the confinement party in your house?
If it is a different kind of other person coming, is it convenient for you to live together?
If your sister doesn't bring anyone else, is it because you want your wife to serve your sister Yuezi?
It is indeed a rare thing to let the sister-in-law serve the sister-in-law's confinement. Such things can also be seen in the script and the play, and I really haven't heard much about it in real life.
Therefore, you still dispel such a thought in advance, and your wife does not agree, which I think is reasonable, understandable, and correct.