I appreciate the thinking, but do they really need/deserve ranking of importance? If it helps, why not have a supportive sister to elp you when you break the news? At the very least, she'll have new or different arguments than yours which may help your parents in their acceptance.
Don't get me wrong, I love my parents and brothers, but when I tell them (yeah im a coward and havent said a word to them yet lol - 1 of my brothers and my mum has realised but the rest haven't cottoned on yet) I'd like to have someone to turn to who I know i can rely on and is supportive of me.
But in the end, its your decision. Personally I feel such tough conversations really make-or-break our resolve to transistion. At the end of the day, if your willing to reveal all to your family and endure that ordeal then you have the willpower to fulfil your dream 😀