She may not ultimately be male, but that doesn't mean I have to cause offence and use feminine pronouns and her deadname in front of her. Maintaining a level of respect for others' feelings sometimes comes above the truth.
So you don't want to offend your sister by using terms she would find upsetting. Fair enough, she's your sister and you don't want to upset her.
But in later posts you say this regarding offence:
I actually had no idea that the gender-critical movement was this opposed to the term 'cisgender', to the extent that some of you seem to think you are agender. Although I now recognise the offence some people take, I will continue to use it because (a) I am simply using it to mean someone who isn't trans, someone who doesn't consider themselves to have transitioned, (b) it's widely seen as a neutral term, and (c) I don't think your offence is reasonable. I am sorry for our disagreement.
And this:
I am not setting out to offend; I am simply not censoring myself despite knowing I might cause offence. You are being hypocritical: the standard you are trying to impose would also restrict you and others who are gender-critical from saying certain things because they are offensive. I assume you would not do that. I can't think of a single forum platform that would ban me for using it--not Reddit, not Twitter, not 4chan; not even Mumsnet (hopefully).
Apparently it's OK to cause offence to loads of women you don't know by insisting on using terms that you know they find offensive.
You're not setting out to offend, but when you've been told that you're being offensive you say you don't think our reaction is reasonable and you're going to carry on doing it anyway.
Perhaps you should have a little think about who's really being hypocritical here.
I'll see if I can find a link to the special rules on language which were imposed on us here about 10 years ago regarding terms which both sides find offensive, since you so obviously have no knowledge of the background of this debate.
I haven't even started university yet (I'm on my gap year at the moment); 'language we threw back at you years ago' makes no sense given my age.
As for this... If you're going to jump into a discussion that some of us have been involved in since you were in primary school, it might be worth looking at the history of this discussion rather than mansplaining your half-baked ideas to us.