NTA - But I'd try not to stress over this too much. I got married in '05 and I've looked at the photo album exactly once since I got it back. Focus on what actually matters and don't get so hung up on silly details. No one is going to think your perfect Disney day is ruined because your sister didn't make her hair one tone.
You could always suggest clip in hair extensions to cover the blonde. If she still says no, then you know she's just doing this to annoy you. If she does say no, shrug and forget it. I promise it won't matter if you just let it go.
Short hair would be SO cute. My vote is for short!
Btw when my hair was very thin (it’s not as thin anymore, but was at one point), I bought something like this (if you end up wanting to keep your length for whatever reason):
12" Clip in Hair Extensions Remy Human Hair for Women - Silky Straight Human Hair Clip in Extensions 50grams 4pieces Dark Brown #2 Color https://www.amazon.com/dp/B078JNXHQX/ref=cm_sw_r_cp_api_glt_fabc_4QPH5PF4R1A5SEJKJ79H