OP cut contact. It is obvious that your brother correctly sees you as the person in his life who is the easiest to manipulate in order become his Forever Support.
You have already discarded normal human reaction to hearing fuckry as possibly being "non supportive" to a manipulative person rather than a normal human instinct.
You are hiding salient information from your probably better boundary having husband , thus indirectly letting your brother affect your marriage whilst he has abused his wife and is prepared to, at the very least change all the dynamics of his marriage, including his children's lives in order to render a sex parahilia a legitimised reality .
Don't listen to the pick me women on this thread. They are on that same spectrum of woman who turn their head the other way when their fella abuses a family member. Not all women like being women outside of making sure to marshall other women and children into facilitating the worst of male intentions to our complete lack of benefit.. Transactivism manages to let the more urbane, usually White, if not, over educated or willing to appear so ,previously needing to be seen as feminist woman express her utter disregard for female dignity and hatred of children, whilst gaining do gooding points.
There is absolutely nothing wrong with cutting contact with a grown man who is obviously co morbidly personality disordered.
Your parents are far from the only ones who wisely do not see supporting infringing delusion and paraphilia as the axis upon which being good parents spins .
There is a reason why Transactivism comes for the education institutions,
Tertiary educated, liberal, Ctorporate LGB and White liberal/left endorsed Male Identified Black Activism.
Amongst each of these factions are a concentrated large amount people who want to belong to 'power", without being seen tas wanting to as this wouldn't skew with their "more rebel than those conservatives over there" messaging.
Don't fall for this Okey doke, OP. The world is full of people who have no relationship with a sibling , a child, a parent and for more benign reasons than getting kicks from imagining themselves as the opposite sex and demanding that everybody around them rewrites reality and then calling it "identity". The world still turns.
I would never permit my child to mentally abuse me and flinging the word "transphobia_" to make me comply would just make me quadruple down in my refusal. Much less a sibling.