I don't want to be transphobic and I don't want to alienate my sister, but it seems I can't share my concerns or beliefs without being seen that way.
I’m mindful of not drifting into hyperbole, but it’s all a bit cult-y, isn’t it? Obviously, it’s not an organised sect with a single leader, but it does function similarly - it feels good, gives a sense of belonging, and the not-quite-explained fight for trans rights (what rights?) feels like fight with a moral and just purpose.
Borrowing techniques from those used by people with family members sucked into niche religions seems to be the most useful practical tactic - ask gentle, open ended questions, don’t let them force you into adversarial standpoints, de escalate as much as possible and don’t let them make you into an enemy. Just chip away, gently.
If all else fails, let them go, but keep the door open so they have someone to come back to.
theconversation.com/how-to-talk-someone-out-of-a-damaging-cult-68930
And you are absolutely right to keep reassuring your daughter re: the limitations of gender roles. If you need to do it in front of your sister, try and frame it as a chirpy, chatty question (eg ‘Aw, you like football, niece? That’s not very ladylike!’ ‘Do you really think so, Sister? Because in America, soccer is most often played by girls, look at the US World Cup team, they are excellent and their sporting brilliance isn’t dictated by how they perform masculinity or femininity, look at all the blonde ponytails! And in Afghanistan, the majority of skateboarders are girls! Etc etc)
Find sideways routes in, rather than confront dead on, if you see what I mean?
There are always people to talk to here, btw, you are never on your own with this stuff. There is an excellent regular poster with a transitioned sibling (albeit the other way) along with lots of mums of gender distressed girls, lots of women who worry that if they were young now, they would be at risk of being convinced into transition, and lots of women who watched their former husbands get sucked into the culti-ness of trans communities, never to return.