My DS was 18months before anything made sense, then it was an over night thing & he just started going in leaps & bounds!! I wouldn't be too stressed, if they are communicating that's the main thing, they will all do these things in their own time.

A little girl 3 months younger than DS at about 16months could copy what her Mum told here & say lots of things & I got really worried DS was behind. But she too had an older sister, so that helps. But now just over 2 he has lots of words & still lots I do not at all understand. And it has only been in the past couple weeks I have noticed he will copy me.

Some kids are very advanced, others just take their time...older siblings can make a difference. As long as you can communicate with them on a level that you all understand then that's fine....one day (like today for my DS) you will just want a little peace LOL & they will be flat out chatting non stop.