Well, it sounds to me like this older girl "may have" siblings or was raised that breast feeding was just a normal part of being a mother. She may not have even considered that to be something sexual at all. Also, if a child becomes aware that parents have sex, it is not out of the realm of including that in child play as children do like to copy what they see or get the general idea of doing, but they may not know what it all means exactly.

It's a good idea to go slowly so your adult mind doesn't see things worse then they were in the child's mind that you and your friends had at the time.