How can we know exactly what anyone elses plan is when they communicate to us? It can be any one, all or a combination of the things mentioned thus far in this thread. Generally, with trading, its about fear. However nice it feels to try to placate their concerns, we cannot control the future, and therefore must understand that there is no utility involved in trying to satisfy someone elses fears about the future. If it breaks up your family to make your peace and then go about your business, if this isn't good enough for them, then your FIL and other similar relationships are just not useful for you, so go surround yourself with people who share similar views. You can think, and maybe act "f*ck them", but you don't have to say it.
If you look at trading as a skill to be learned, then you know you can succeed. Make reasonable provisions for living well as you learn your skill, and treat what you consider victory and failure with the same face (this works well if you have friends and relatives that dont understand, and it helps your trading...remember your edge is produced/revealed over time of many trades...not any one). If you are married, plan it with your wife, since she will be involved with the risk and reward. If you truly understand what risk is, you can teach it to her as you learn to think in probabilities. Make a simple statement to you FIL that communicates your respect for him, but equally important (if it is so) your desire to become a career trader. If you are going to waver here, how can you expect to hold your own as a trader?
Ask yourself, if you were making a great living trading, how would that change their perspective and support of you, if it is too different than now, it may be very important for you to find some trading specific role models, as you have begun to do here at least in thought. Find creative ways to keep communication open with your in laws, but remain focused on your goal.