Well I've had a look into my dream book and I'm going to take a stab because I feel unqualified to answer, but also wouldn't want someone to leave a thread of mine they saw without replying!
So,
1. You're trying to escape.
2. Your sister's escaped.
3. The figure is intimidatingly big.
4. Your sister is encouraging you but not actively helping you.
5. You're struggling out but you don't feel like you're moving anywhere.

I would have said that you're in a state of mind where there is an anxiety being kept at bay that, although has not struck, is lingering in the back of your mind no matter how hard you try to suppress it. Your sister having got free and giving you encouragement is perhaps because you admire her for not having the anxiety you have - like a "nice" kind of jealousy or envy. I think the dream is looping because it has not been faced, or dealt with. I don't think the shadow is a person necessarily, but in general a bit weight on your shoulders.

Is there anything you've always struggled with, but shrugged off instead of overcoming it? Is there anything you specifically find inspirational or something to aspire to in your sister?