My formerly estranged brother has transitioned and wants back in mine and my mother’s lives.

Blood isn't thicker than water, you don't have to have a relationship with him just because you share the same parents.
If you want to support your mother, that's great but be sure the support she's asking for suits you. It's her choice if she lets him renew their relationship but she can't make your choices for you.
I'm sure you can find a way to support her without having anything to do with him, which if it were me I wouldn't, I'd be telling to get lost, but that's me, not you.

You don't need to have a relationship with him or involve him in your family's lives. You don't need to talk to your childern about him.

It shouldn’t be about what he wants!
So what if the callous Brother has now decided they want a relationship. Put your needs first OP.

Well he's a shit, isn't he, getting his compliant army in first. I can see why she feels under pressure. And he is her child, she may see this as her last chance. But you don't have to play along if you don't want to, just say 'that's nice ' and move the conversation along if she tries to talk to you about him.

Stay estranged and save yourself a whole lot of hassle. He's no doubt after a compliant audience to play long in his theatre of life. Wide berth. VERY wide berth.

A dick in a dress is still a dick.

I would just say, to me you are my brother and always will be, I am sorry if you find it offensive but I can't address you any other way. If you want to be in my life you are welcome but it will be as my brother.

Then if he doesn't accept it is his choice. And got have done nothing except assert your personal boundaries.

“He wants involvement”

Oh does he indeed? Well, you don’t have to let him have it. You can be supportive to your DM and civil to strange brother when family occasions mean you can’t avoid him, but otherwise, I’d carry on just as you are now. I see absolutely no point in trying to play happy families with a horrible person who has been estranged for some time, and I’d want to keep my DC well away.

I'd be very suspicious of the timing of all this, waiting until after your father's death (a vulnerable time for you and your mother) to make his announcement to the wider family, only then saying he wants to be accepted back into your lives.

His 'transition' doesn't erase whatever happened to cause the estrangement, nor does it obligate you to forgive and forget if you don't feel like it. I probably wouldn't be eager to allow him back into my life, personally. Your mother can do what she likes, but I wouldn't be guilted into doing something I didn't want to do. The rest of the family can think what they like.

So he’s manipulated other members of your family to make mum feel pressured to extend the olive branch. Or has he tested the reactions he received from other members of family to give him the confidence to meet mum?

Oh for goodness sake! I’d be furious about that, so manipulative.

Oh right - then in that case I wouldn’t give this any more thought.

Just reiterate to your mum that you are there for her and her alone.

And don’t ever be tempted to have any kind of showdown with someone like this.

oh and keep your kids away needless to say

It’s sad she is definitely feeling the social pressure to make nice and be tolerant.

It will be because she is a mother and so relieved they’re not estranged anymore, not societal pressure.

i have a brother I haven’t spoken to or seen in 10 years. My mother still sees him on the regular. Initially she went on and on about how I was making things difficult for her but I just held my ground and said I wasn’t stopping her seeing him but she had to respect my decision to no longer engage with him. My brothers’ not trans but the same approach applies. You can support your mum but that doesn’t mean you have to have anything to do with him. If you were estranged before I’m sure him popping a frock on hasn’t erased the issues that caused the estrangement.

This is nonsense, sorry. If someone 'coming out' on their own terms makes them a shit, then good lord the bar is low. My brother came out to our aunts first, then friends, then me, then our sister, then mother, then dad and then grandparents and wider family - thought there are still some he's never bothered telling because they're homophobic scumbags. Doesn't mean he was building a 'compliant army' for crying out loud - just being sensible and safe with who he told he was gay. Coming out as trans is not different from that perspective.

People can want whatever they want. Other people are not obliged to give it to them.

To be fair, the way the Op said brother went round and secretly canvassed all the extended family coming out to them first and telling my mother last. certainly indicates that she also thinks there was an ulterior motive

If your dad’s death is recent I think it’s reasonable for your mum to say she is not in the headspace to meet up yet.
There is no rush and a way to support her is to ask family to allow her time.
I would suggest when they do meet it would be outside the house until you’re sure it’s a good relationship for your mom.

He could be wanting a relationship now as he felt unable to when your dad was alive. He may of felt your dad wouldn’t of accepted him and now he is gone he feels he can be himself with you all.

So she’s being nice and accepting to him because of society pressure rather than a mother simply over the moon her son is back in her life?

Ok 🤷‍♀️

Do you think he might ask her for money? I am glad you’re there to support your dear mum. Just be cheerful and supportive and let her talk about what he’s like etc when she does see him. Play the long game x

That's very telling isn't it? There's lots of good advice on here - one thing I'd add is that if he's already spoken to other extended family ,members but not you, he may be setting you up to be the baddie? Which is another reason to grey rock him. I think someone suggested "polite indifference" .

You can be polite, say nothing that would allow him to weaponise and focus on you and your family. If further down the line it becomes apparent that he's genuinely remorseful for his previous callous behaviour and isn't spending time policing people's pronouns etc, you can always change your approach.

But you were estranged for a good reason.