"She is coming inside."
Translation:Sie kommt herein.
Your german sentence only makes sense in a sexual way.
Kommen like in the english sentence describes a movement from one point to another (even when not both points are directly mentioned). Shes coming inside. She is moving from the outside of the building to the inside of the building, to create a sentence in german you would use herein. Drinnen describes something that is just inside. She is cooking inside. Sie kocht drinnen. Theres no movement involved and its somehow stationary, this would be described by drinnen.
How about "nach drinnen / nach innen" ?
nach drinnen would be just about possible (and is accepted), but I would strongly prefer (he)rein.
nach innen is not appropriate; innen is more for things that are attached to the walls rather than the entire space between the walls.