Have you thought about what will happen when you want to baptize your babies, and DH objects because they’re not “old enough” and then wants his brother to baptize them with a “believer’s baptism” when they are “old enough?” What about when you want to put them in RE or Catholic school and have them go to Confession, receive First Eucharist, be Confirmed? Baptists don’t believe in the Sacrament of Confession, they don’t believe in the Real Presence in the Eucharist or the miracle of transubstantiation. If he wants to remain attached to his denomination from the very beginning, he will likely fight you on these things. Also, Baptists don’t believe that marriage is a sacrament. They believe divorce is undesirable, but allowed, with ability to re-marry inside their church intact. He will not have the same view of your marriage as you will. That can cause strife. The Baptists I have known are actively hostile and derogatory toward the Catholic Church. That would be very unhelpful in raising Catholic children.
In short, if you want to be married in the Church, I presume you want to have a Catholic home and family. You may not be able to fully have that, and you may end up with religious war in your house, if you marry someone with radically different beliefs.
This is a central, major issue, and should not be taken lightly. Love is not enough to make a marriage work. Read some of the threads in these forums… there are tons of people who had promises from non-Catholic spouses to support raising the kids Catholic who are now dealing with the consequences of that agreement being broken. People think it’s not that important, but then find, especially when children come, that they cannot so easily let go of their own traditions and upbringing.
This should be your focus, not the logistics of who stands where or does what at the wedding. Though, I do not think your BF’s brother can officiate. My IL’s had their wedding at her Presbyterian church, but with a Catholic priest “co-officiating.” We are now dealing with the aftermath of that in that as my parish can find no evidence that they are in a valid Catholic marriage, they may not allow them to be godparents.

It seems that there was no dispensation for this wedding, and no certificate from any Catholic parish.