GC for parent - can my sister claim to be dependent?

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I am a USC and I filed an immediate relative I-130 petition for my father. My father remarried about 10 years ago, and adopted a child together with his new wife. Wife and child are listed on the I-130 petition that I filed. I understand that once the petition is approved, my father and his wife can file I-485 together. His wife would qualify as a "dependent" of my father. My question is, what about their 9 year old child? Can she also file I-485 and claim to be "dependent" on my father?

Or is my only option to file an I-130 petition for his daughter, because she is my half-sister?

Thanks in advance for any answers!