I second pushing the liquids but also so you stop freaking out, lol, over the years, it seems that some babies have a greater propensity for the soft spot looking sunken when it is just the shape of the child's head or that the soft spot is slower in "closing up," which is still totally normal because children develop in all ways differently even among siblings.
Another word of advice, sometimes people tell us things trying to be helpful when they often don't know what the heck they are talking about and don't stop to think about how badly the information is going to scare you, etc. You did the smart thing in calling your cousin and I wouldn't assume that anything is wrong.
Try to keep the child more hydrated and keep an eye on it, if it worries you, take the child to the doctor, sometimes that peace of mind is well worth the price of the doctor's visit.