Emotional is in relation to feelings or emotions. It is a state of being. Sentimental is a type of emotional state of being. Sentimental is a state of nostalgia or tenderness. For example, let's say that you had a boyfriend in high school. He was your first love and the first gift he gave you was a teddy bear. Now, you have been broken up with him for ten years but you keep the teddy bear. You don't want to get rid of the bear because it gives you nostalgic feelings that make you remember tender emotional moments with your ex-boyfriend. That is what we call keeping a momento or sentimental item.

I think of 'sentimental' as a particular kind of emotion. An emotion that is typically used to describe an excessive fondness for something in the past. Thus, someone can be 'emotional' (have an excess of feelings) but not be 'sentimental'. Hope that helps.