B2 Visa Proof of Accommodation Request

Hello,

My family and I are planning to visit wife's brother in the United States. We are a family of three, my in-law lives alone and he has a very spacious 1-bedroom apartment that can take all of us.

Now, for the proof of accommodation, will it be better to make a free-to-cancel hotel reservation or will a letter of invitation from my in-law stating that he is willing to accommodate us be sufficient?.

My major concern is since we are a family of 3, will that negatively impact our application, since my in-law's apartment is just a one-bedroom apartment. I know he is not required to state how big his apartment is, but I am trying to be really careful here to avoid a refusal.

Any help with this will be really great.