There is no question that you can crowdfund to help your brother and his wife.  An attorney general who looks at the situation will want to know that the funds are being spent for their benefit, but it is clearly legal to ask and transmit the money you receive for their benefit.

The legal issue arises because such gifts are not deductible as charitable contributions by the donors.  A deduction is not available for a gift to an individual, only for a gift to a charity.  Even the gift you describe to the church for your relatives’ benefit would not be deductible because they are going to a pass-through fund earmarked for the relatives.  The IRS treats that type of gift as a gift directly to the individuals and therefore not deductible.

Gifts to recognized charities for disaster relief funds run by groups like the American Red Cross, a community foundation or a United Way organization would be deductible so long as the sponsoring organization makes the ultimate decisions on how the funds will be applied to assist the individuals. 

You can’t assure that the donations would be used for the benefit of your relatives if they are made to a disaster relief fund, so you should do what you are planning if you want to assure that the help goes to your family members.  Most taxpayers don’t itemize charitable contribution deductions these days because the increase in the standard deduction eliminates a direct tax benefit from the contribution so it probably won’t make much difference in how much you can attract in donations.  And you will know that you did what you could to help your family in this tragic situation.