Hi Jose, welcome to the community!
At the start, I think some relevant questions are:
1. Do you live near your brother, or are you trying to convince him to invest with you long distance?
2. Have you looked into the logistics of rolling over a 401k into a self-directed IRA, and do you understand how to partner with an IRA without violating the prohibited transaction rules?
3. What is your experience level with real estate investing? Real estate in general?
As a side note, it sounds like the main concern should be sounding sane, since your brothers think you are crazy! ;)
Putting the 401k aside for the moment, I think the first task you have is to show your brother(s) that you are serious and you have done your homework. I don't know your experience level, but if you haven't done any deals yet, it could be difficult to convince someone that you know what you are doing.
You may also have to help a new investor understand the basics of real estate investing if they don't already. Perhaps you could pick out the episodes of the BP podcast that you found most informative and inspirational to have your brother listen to.
Another important point is to show what you are bringing to the table that adds value. Are you contributing money also? Are you an experienced construction worker who is going to be doing the renovation work yourself, and thus cut down on labor costs? Are you already a licensed sales agent with experience marketing property? Do you have great credit and enough income to help qualify for a bank loan? Perhaps the value you add is simply enthusiasm and the depth of knowledge you have based on your research. If so, you have to show a potential investor why that will be a good enough reason to trust you with their money instead of a regular stock market index fund (especially since the market is performing well right now).
You can never convince someone to do something they don't want to do. You have to make them want to do it.