Stormy's right! (Why am I not surprised? )
It sounds like you and your dad, as two role models to your bro, should sit down together and discuss the whole situation. It also sounds like you have a really good handle on the different aspects of it, and will be able to talk to your dad, and your brother if that is what you and your dad think is best, about the risks and benefits of medical marijuana.
I am of the strong opinion that the line between medication and abuse is defined early on, and that reinforcing the fact that one should not use substances recreationally that are intended as medicines is a very good way to approach this. So in your conversation with your father, maybe you two should brainstorm, honestly, your own reasons for toking, so that you have a good case for yourselves. Do you see signs of anxiety, insomnia, or ADD in your own person? These are SO common among adults, that chances are you can say honestly that yes, you do receive positive medical benefits from it, and are aware of that fact, and that is why you choose to continue using it.
14 is a bit young in my opinion too, and I think that even if a young smoker DOES have a legitimate need to medicate that way, it is very important that he has some guidance from a parent or medical professional- it is hard for minors to make these judgements for themselves, not having had lots of world experience, and also since the hijinx of teenagers often lead to arrests and possession charges that can put a financial strain on the family and interfere with their number one job, which is to get an education that positions them to live a happy and self-reliant life.