Agree with Seaweed.
Give it time.
Having a huge death /disease situation, is not only stressful for you but probably she was stressed out over it as well. If she wanted a couple fish too, she was likely sad about loosing the others.
Best advice here I can think of, is don't pout, and for now and the future, find common ground with those around you and built off of that.
Don't always be thinking of yourself first. Here is the example, you had disease outbreak. Stocking a tank with fish numbers in the teens would likely stress your tank anyway, causing a bad start. Think of your fish...
As for the stepmom, be respectful and kind. Having an underlying anger or hostility will only be disruptive. And asking around and going to dad will be showing disrespect. Let it be for a little while.
She likely saved you from a mini cycle. When you are young weeks seem like an eternity, but not true...after the time of a few weeks go by and you still have a stable tank, no illnesses, deaths... Then just start by being smarter and advocating for your position from a positive perspective.
Look the fish are doing great...the tank parameters have been perfect for 4 weeks..., etc. After lots of positive words and happiness, start taking up her favorite fish, then make notes and show how this fish or that fish are schooling fish or shoaling fish. Hey since she has fish in there show her AQ advisor about tank stocking and how cool it is you can + or - fish in the tank set up to know what will work or not, how peaceful ones work together, etc.
Also know her communication and care style. Like it would make her sad if the fish was lonely, she is an emotionally driven person. Fact based cares highly that the parameters are good and the tank isn't overstocked...you probably get my drift. Direct your appeals, conversation to speak to her interests and priorities.
Get her interested on your behalf and you will be able to enjoy this fish keeping thing much better. And the wicked stepmom might not be so wicked after all.
That said, if the building and strengthening relationship thing doesn't work out, my thought about it would to be to wait a month after the last NO FISH stepmom comment and then approach dad with a plea of fish care requirements, like X fish require a group of 5... I know we didn't want to get too many fish but I can tell by the behavior that it is unhappy...use facts only (no comments like she is so mean and hates me...and doesn't want me to get any more fish) and ask dad to present the case on your behalf. Again only after the tank is stable and doing well.
Hope this helps.