I agree with the others. All babies develop at different rates, and in different ways. Some are faster with physical milestones but slower at talking. Or vice versa. Even with siblings there can be big differences as to when milestones are reached.

I also agree with trusting your instinct. In this case I suspect that it is only your MIL's comments which are making you worry. In which case I would ignore your MIL - if my experience is anything to go by, MIL's are quite often wrong about what is and isn't normal with babies - they really seem to forget. But, if it is something that you "feel" yourself, that there might be a problem, then I would investigate further. Hopefully the chiro will make a difference to start with so that you can stop worrying.

Best of luck hun.