With a sibling who is trans and who may be adamant about needing blockers, or who is even further down the road, I believe it is soul-destroying to realize that this path may be wrong. So I don‘t think you can change her mind, I‘m not even sure you should.
When everything is said and done ( like e.g in the case of Jazz Jennings), I believe it is best for everyone involved to believe they did the right thing, if they can manage to do that.