Although I realize this may seem over the top, I am saying it because it certainly is something I WOULD DO (even at the expense of risking a relationship with family) if I had a 9 month old nephew about to be circumcised...
I WOULD calmly call the local authorities and explain that you want to intervene legally as an advocate for your very young nephew who is about to be subjected to a criminal sexual assault. Tell them you would like them to protect your nephew from this non-therapeutic and damaging procedure.
and explain the reason why MEDICAID NO LONGER covers this awful cosmetic surgery...
NOW...
you would definitely expect at the very least, some resistance from the local authorities...
even more so, you might just as well expect them to think you are some crazy woman (and I would have to assume you will be talking to a majority of circ'd males...sensitive topic for them, whether they may admit or not...KWIM?) that is why I say present your case calmly and with scientific facts.
Understand THEY will more than likely NOT support you and probably WILL NOT do anything at all...
BUT...
1)You can say to your nephew one day that you did EVERYTHING you could that was in your power to stop this
2)You can confront your brother and SIL with the strength of your character and tell them you called the police in an attempt to have them intervene.
MAYBE just POSSIBLY they will think "Jeesh, she is really so passionate about this that she would try to have local law enforcement get involved to protect my son..."... it may very well drive them away from you, but it JUST MAY OPEN THEIR EYES to what a crime it is!
It is a risk I WOULD ABSOLUTELY, MOST ASSUREDLY be willing to take.
I understand if some would think I was being over zealous... but this goes back to another thread about certain forms of activism being too harsh...
This is not just some stranger you are trying to educate. This is blood.
I personally would do whatever it takes, within legal parameters, up until the last minute, to try to change what you may now see as inevitable.
Try to get your brother to protest against this!
I wish you the best and will be thinking of your nephew as Friday nears.

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