Does a verbal agreement for a car loan hold up, when the title is in only your name?

I purchased a car roughly a year ago, for $1900, and my parents loaned me $1700 for it. Recently, my brother moved back in with us, and is looking to take the car, and he is talking about buying it for only $1300, and refuses to give me ANY money for what I put into it (an extra ~$400-$600), saying it is "lost money, because it was used for repairs". He is also planning to try to get my dad to take the car from me, even though the title is in ONLY my name, nobody elses. He thinks that just because I was loaned $1700 to buy the car, my father/parents have legal possession of it still, and can take me to court if they choose, and force me to sign the title over to my brother. Would the fact that the title is in only my name override that ability, as it was only a verbal agreement?