Interesting question. I'm not in anyway an expert, but in my interpretation YES your child is Jewish.
If the Bgramma was then the Bmom was (even if she didn't think of herself as Jewish), and if the Bmom is then her kids are. The adoption doesn't negate that in anyway under Jewish law.
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Jillian
Anabel's mom
Daughter's DOB 4/18/2006
Receive referral 6/1/2006
HOME AS A FAMILY 12/23/2006
June 2009: Officially waiting for kiddo #2!
7/2009: Profiled - decide it's not the right fit for our family. Back to waiting!
10/2009: Profiled - not selected. Back to waiting!