@gum_rummy eh, I wouldn't throw around "grooming" so easily. Without getting super detailed (and based on my own limited knowledge), grooming primarily deals with an adult raising/conditioning someone young to accept certain behavior and actions that society (and really any normal person) would find reprehensible (i.e., an older man grooming a young girl to accept physical/sexual advances). In this situation, she is trying to form a bond so he protects her and/or doesn't want her dead. Plus, as others have pointed out, she may have her memories of her past life, but she's got a baby brain and all the baby brain chemistry that it entails. I seriously doubt she remembers most her life and probably regressed some due to the baby brain (maybe kind of like adults with Alzheimer's?). Eventually, her past life will play some part, but it would be hard to argue that the ML wouldn't mature fast and match her maturity when you consider the fricking environment he is in.
As for them still being siblings despite no blood relations...if they grow up not viewing each other as siblings and are eventually raised apart, I wouldn't consider them siblings when they share no blood. It comes down to how the author executes this if they get together (i.e., when does the ML realize she is not his sister, what age does it turn romantic). Personally, I believe consenting adults can do whatever they want with each other so as long as any romance doesn't start early, I'm staying for their cuteness. Hopefully, we get alot more baby shenanigans.