Remember there is a very "broad" range for meeting developmental milestones. Also odd behaviours at 18mths, is pretty early to be getting worried. When she turns two, rethink it. As her care provider all you can do is provide her with opportunities to grow and learn. If her parents aren't worried, at least you can say you spoke up.
I have an almost three year old with delays (obvious to me), but mum is not worried. When he starts school, if he still has delays, they will be caught. I'm just doing my best giving him opportunities to practice speaking, repeating after me, using fine motor skills, interacting with the group. It is definitely NOT my job to diagnose. It's is my job to take good care of him and provide him with opportunities to develop.