It's hard to say why she watches the older dog eat and then barely eats when he is finished. Are you feeding the same food she has was eating before you got her? What was her eating schedule before you got her, maybe she needs a little adjustment. Dogs are creatures of habit.
Do you crate train? I feed my puppy in crate, it worked out great, she would eat and then when she was finished I knew exactly where she was when it was time to go outside and potty. I would take up her food after 10 minutes (actually probably 5) and not feed her again until the next feeding time. I would first try and find out about her feeding schedule before you got her and make some adjustments from there. If she was being fed twice a day then try feeding her in her crate and only giving the food for 5 minutes. Good luck