Getting permanent visa for minor who I am the legal guardian

[Kanchu]

Hi,
I am a permanent resident holding Green card in US. My sister died recently in India and I am the legal guardian of my sister's 6 months old daughter now. I cannot adopt her according to Indian laws since i already have kids. So, I need to take her to US with me but as her legal guardian. Is there any way that I can take her with me? Like Family-based green card or any permanent visas? Please post if you have any info. Thanks