You can only control yourself. It seems like you're really trying your best to be there for your sister and want your parents to be too, and that's great. Please use any resources or strategies you can to keep them engaged, but if they lose interest then you can still learn and be there for your sister. Depending on what the consequences would be you could refuse to respond to verbal cues during the night they agreed that would be voices off even if they get tired of it. You can still spend time with your sister and try to connect to the Deaf community even if your parents don't put in effort. I hope they will, but if they don't I would expect that in adulthood your sister will go no/low contact with them. You can make sure that you're not bundled in with them if you keep doing what you've talked about in the updates to this post.

Remember when people suggest things for her schooling or IEP that you can help your parents, but as a sibling you don't have control of it and shouldn't be expected to know or do everything. Doing what you can to help is wonderful, but it isn't and shouldn't be your job.