Sorry to hear about this, Fitz.
I'm assuming your aunt has either power of attorney for, or guardianship of, your grandmother, otherwise she shouldn't have been able to place her in a nursing home to begin with. If that's the that case, then unfortunately she does indeed have the right to do so without compromising with any of her siblings about it, or even notifying them. Your mom can speak with the nursing home ombudsman if she has reason to believe your grandmother is receiving inadequate care there, but other than that there's probably very little she can do. Is there another sibling who actually wants and is prepared to care for your grandmother in their own home, or is the problem basically just the failure to discuss the nursing home move first? Hiring a trained family mediator might be something worth considering if this situation of neither side feeling willing to talk to the other continues.
I'm the guardian for my mother (she's brain-injured and lives in a nursing home) and have been fortunate that there haven't been any serious disagreements with my siblings about her care thus far. It's not necessarily evil for the child who has legal authority and has been the primary caretaker to place their parent in a nursing home despite some siblings' disapproval, but making and executing that decision in secret ostensibly to avoid confrontation does seem worrisomely irresponsible, and abusive towards the siblings on a personal level.