Concentrate on this fish you have before you start thinking about the fish you want.
I deal with customers like your sister on a daily basis. Someone came in and fought with me one day over how they could keep an Oscar, a Blood Parrot, a Black Moor (Goldfish), a couple of Bala Sharks and six guppies in their 90 gallon. I told them, basically, "Yes, it's all fine and dandy now. But that Oscar is going to go crazy and kill everything." The customer stared at me in disbelief, and then proceeded to tell me about how "docile" and "sweet" their Oscar was. Yea, okay.
I have people fight with me over how big comets get. They are feeder fish, so we sell that for $0.13 and $0.27, and people want to tell me that they DO NOT get a foot long, so therefore they can live in a 1/4g bowl for their entire lives, or 3 of them can be crammed into a 5g. I've learned the best thing to do is to give them what they want as soon as they start fighting with you. They're not worth my job, and they're not worth my time, and, unfortunately, my manager won't let us refuse sales. I've got tons of stories like this, including one about a male betta and two females apparently living in harmony in an undivided 10g for a year. Ask me about my thoughts on that, because I wanted to snatch the female betta they were buying away and send them out of the store.
The best thing to do with your sister is let her do what she wants, as sad as it is to say. She isn't going to listen to you. The goldfish is going to be the culprit at taking everyone else out.
One) Goldfish are dirty, dirty fish and create a lot of ammonia. Ideally, goldfish in a bowl need their water changed one to two times A DAY. The ammonia is going to build up, so the combination of all of the ammonia the goldfish is letting off the ammonia from the other fish is going to result in ammonia poisoning.
Two) Depending on the type of goldfish, it's going to get between 6 and 24 inches. The most common goldfish, and the cheapest, is a comet. They get 12 inches and will need 24g of water all to themselves when they are fully grown. Most of the time people can get away with 20g of water, but it requires a lot of maintenance.
Three) Goldfish are greedy and will basically eat anything that will fit in their mouth. Any other fish in the bowl with that goldfish will become a snack eventually.
Just let her have her bliss in her ignorance and refusal to learn. Eventually, she won't have any fish.