Does she want us together or not?

It is hard to tell. You shouldn't keep going behind her back for much longer though. Maybe about a month or so, to see how things go? After a month, if he hasn't asked you, ask him out and let him know that you will be together even if his sister doesn't approve. After then, you and he sit down with her and tell her you two love each other and are going to be together. See what she says and try to work things out among you three. Good luck!