From a very similar experience, I applied for my sister's tourist visa to help us babysit our child once she was born, but my sister was denied a visa. In her DS160, she only had estimated arrival and departure dates (no plane tickets backing up her round trip), I provided our address as her place to stay, and that we planned for her to stay for 4 months. Though she was going to come to the US for the purpose of babysitting our daughter, we believe that the denial did not come from that; rather, it may have come from the fact that we mentioned in her DS160 that my sister would be planning to stay in the US for at least 4 months, not to mention she was also going to stay in our residence. The first and only question that my sister was asked before she was told that she was not granted a visa was "how long are you planning on staying in the US?". To the interviewing officer's judgement, my sister's case was easily a strong case of an applicant with an "intent to immigrate or overstay," even though we are truthful and honest about her arrival here as a "babysitter" for just a few months. I hope my ignorance gives you a little bit of an idea on how you are going to fill out your sister's DS160 if filing as B1/B2 applicant.
Best wishes!