Trusts are contracts. You don't necessarily receive money when you are 21, you receive money when the trust says you do. You need to see the trust to see its terms. As a beneficiary you have the right to have information about the trust and the money in it. If you are not speaking to you grandfather you should hire an attorney to speak for you.
It's a crazy thing about Florida law, homestead passes outside of probate, but you have to hire a probate attorney to get title to it. Since homestead is not liquid I require a client (the PR) pay me up front to probate the estate. Your brother is suggesting you pay a proportionate share of the fee as you are deriving benefit from the probate process. It sounds fair to me, although since your brother has more money he could afford to pay for everyone. Consider hiring your own attorney...
You need to hire an attorney to prepare or review your agreement. When money is at issue hiring and paying an attorney is a straight-up business decision. You want to protect yourself? Hire an attorney.
Hire an attorney to help you. The attorney is paid on a contingency fee basis. Insurance companies do not step forward on their own and pay your bills unless you assert you rights. Hire an attorney to protect and assert your rights.