If they are separate reservations (your immediate family has a room, and the in-laws have a separate room), then as I understand it you can only make FP reservations for yourself the first night and then their window will open up the following night.
However, I noted a possible glitch/workaround earlier this month when I was making FP reservations for my large extended family group. Some of our group is arriving two nights earlier than the rest of the group. While I couldn't make new FP reservations for the late-arriving group until two days later, I did note that I could copy existing FP selections from individuals in the early group to members of the late group. Then, once I had the same 3 FP copied from early to late groups, I could go in and change the FP in the late group to whatever else they wanted to do (but only in that same park).
Dunno if that was just my lucky experience, or if that will work for everyone in a similar scenario.
