I saw on the internet that a prominent forehead vein on the forehead could be caused by a sinus issue, would this be true? the forehead vein bulges

Odds are: This is just a variant of normal. The are deep veins not to far from some of the sinuses, which when partially obstructed can lead to more prominent veins of the face but this is very uncommon and usually presents with a severe headache if not stroke like symptoms. Talk to your parents about this and you may need to review this with your pediatrician.

Probably normal.: Forehead and facial veins usually enlarge when bending over or straining or smiling. Although they could possibly bulge from a sinus infection or blockage this is not common. The reasons for the change in size of the veins are many including changes in intrathoracic and intra abdominal pressures, levels of hydration, and positions of the body. Most likely what you experience is normal.