Hi, Heather.
I am very happy to share my little knowledge with you. As DJ said earlier, the incidence of familial moyamoya is very low, but the literature is only reported in Japanese peoples. In Korea, sorry we cannot have the exact data, however I can see the some cases of familial moyamoya and feel that the incidence is very rare too. Also the chances to have the moyamoya in the siblings are much lower than the mother has the moyamoya.
In Korea, most of the neuroradiologists are familiar with the findings of the moyamoya, so it is no problem to diagnose the disease correctly. The tests of MRI/MRA are enough to see if your baby has the moyamoya. It needs for her some sedations (but, it can be a stress to your child) and you can test in 1-2 years of age if you want earlier to see it before the symptoms(TIAs) develop.
Also, in here most neurosurgeons do not wait for the future developing strokes. If they see the moyamoya, most surgeons lose their patience to wait how it goes further. But I think, the moyamoya kids generally have more potential to adapt their brain to survive than the elderly moyamoya peoples... If your child gets the MM, please consult to Dr. Steinberg or Dr. Scott or Others in U.S. for second opinion.
I think that the MM is not the surgical one even though some cases have markedly reduced blood flows having difficulties to overcome that situation. The next step is that all the folks in this board & all the medical doctors concerning the MM have the key.... and wish to solve the problems together.
I wish you can have more advices from here about your decision making process and I would willingly to join that.

And If you want more talks, feel free to call me (my cell phone No. is
018-311-1128)
Thanks.
Jin-Il.