I would be reassuring DC that despite what some people seem to think at the moment, it's still legal to have freedom of thought and that they don't have to believe in gender ideology to be respectful to their step-sibling.
I would also warn them that if anyone suggests that the views they hold around biology and gender are bigoted, that's not because they are bigoted, and it's not even because those people think its bigoted - it's because they have been told its bigoted by people trained by Stonewall, who have all the power at the moment (as they dictate most major public bodies and private sector policies on sex/gender).
I would show them the Stonewall glossary (especially the definition of "transphobia", which suggests that even thinking that sex occasionally matters is transphobic) so they can get a feel for how totalitarian the views that have seeped into society in the name of "equality" are.
As a PP suggested, also reassure them that the courts have confirmed they are entitled to think gender critical thoughts without fear of persecution (the Forstater case), but that Stonewall and its supporters and the organisations who pay it to lobby them disagree. Point out that Stonewall's view is merely one view and that the reason people parrot it is because Stonewall's voice is so strong and authoritarian, not because they think DC are being unreasonable.
I think the hardest part for young people is being socialised into feeling deep shame around believing that biology is real, so it's important to reassure them that it is acceptable to believe in biology, although lots of people have forgotten that at the moment as they've been socialised to believe questioning anying Stonewall says is bigoted by definition.
I'm sure they wouldn't be unkind to the child anyway, but it's worth mentioning that they should be cautious when discussing the issue with friends or the parents, etc as many police forces have had their policies written by Stonewall too, and because of Stonewall's Workplace Equality Index, those police forces are incentivised by Stonewall to take action against what they believe are "transphobic hate crimes", because they're struggling to understand what transphobia means too because Stonewall has advised these organisations that the law is how they would like it to be, as opposed to how it actually is.
That doesn't need to mean don't speak, of course, but I think the bottom line is to remind them that rejecting gender ideology is perfectly compatible with treating others with respect and compassion, and that people are very confused about the law and whats acceptable at the moment, but that there is a growing movement of gender critical voices working tirelessly to make sure they grow up in a world where biology isn't a dirty word and that they retain sex-based rights and freedom of speech in the eyes of the law and society.