Yes, it is a valid concern. My daughter is 18 months older than our baby and while she loves her sister like crazy, she can't always control herself to be safe around the baby.
I've caught her throwing blankets into the bed because she knows they are the babies, she's dropped toys on the baby's head to help soothe her, etc. Don't get me wrong, I'm with the kids all the time, but this girl is quick!
My babies DO share a room and share it successfully. The older is 24 months, the younger is 6 months old and it's working like a charm. In fact, when considering a move to a 3 bedroom house I'm torn about whether to separate them!
My doctor's take on it was to just be very aware of the concerns but to move ahead if we wanted to. Since moving the babies in together, the older baby has become much more cautious and the younger is crazy about her sister. No joke, big sis gets a smile and a hug from the little baby before I do after naps!