All kids are different, and it could be completely normal, but follow your parenting instincts. If you're concerned, then it certainly wouldn't hurt to have your older child's speech and language development evaluated. If you're in the US, public school systems will do evaluations for free starting at age 3.
This is a helpful and informative web site:
Speech and language development in infants and young children Caroline Bowen
FWIW, my DS was speaking very clear sentences at 2. DD started talking a little later, but even so, something didn't seem "right" to me, despite the fact everyone around us told us she was fine. We had her speech evaluated at 3 and my concerns were confirmed. Although she had a very large vocabulary for her age, her articulation skills were delayed. She went to speech therapy from age 3-6, and blossomed from a shy leg hugger to a social, chatty butterfly. Currently 9, you would never know she ever needed help with speech.