Sentimental is someone who gives great importance to feelings, like a romantic person. You could say someone is sentimental if she or he cryed over a silly movie, or makes a dramma about little things. A sensitive person, although, is someone who is, like Fanahan Barry said, more aware of things happening around you, aware of other people's feelings, but the word "sensitive" is not only applyable to people: you could have, for example, a sensitive button in a remote control, which you wouldn't need much strength to press, a light touch on it would make it activate. Sorry if this isn't as accurate, but I hope it helps you.

Sentimental means that you often think about events from the past - usually with a meloncholy feeling. Sensitive means that you are more aware of a) things happening around you, and b) of people's feelings.