How to focus multiple faces with 35mm 1.8G

I'm relatively new to and trying to get my head around many concepts so please ignore if my question is stupid.

I wanted to take a family portrait but wasn't able to get all faces in focus. The one issue is that I do not have too much light the room and is an indoor setup.

What f-stop/ISO/focus etc would you recommend. Not sure how the D90 actually managed to pic the focus point. Any help or pointers to articles etc would be really appreciated.