(a) Most of the milestones out there should be looked at as "ballpark" age ranges. Like others have mentioned, all children are different in their abilities, interests, and developmental focus. And like you said, she is still working on crawling/pullingup/walking, so that (motor skills) is her focus right now. One milestone that I would keep in mind though is to make sure that she is saying at least one word by one year old (but hopefully a couple words in addition to mommy and daddy by one year old). If she is looking at you, trying to imitate you, trying to make noises, and otherwise interactive with you, she will probably be just fine but might need a little encouragement to learn to say words and to learn that saying words will help her get what she wants. Praise her for all of the sounds that she makes (even raspberries, etc), talk to her constantly about everything you are doing, and imitate back to her everything that she says so she learns what imitation is all about. The more she starts imitating you, the more she will practice language, and the more she will start to say on her own. The twin phenomenon can delay speech development for various reasons. Most children who develop language at a slower rate than the norm end up just fine in the end, but it is absolutely beneficial to get a speech therapist in there to work with you and your child as young as possible! IF she needs it. The quicker she can catch up, the quicker she can move on in all areas of development. AND, the less frustration you will all experience as she moves into the "terrible twos".
(b) I am an early intervention speech language pathologist, which is the type of specialist that your child would likely see if she needed the service. I would definitely recommend getting a hearing evaluation through your pediatrician, and then ask your pediatrician about or contact your local Infant and Toddler or Birth to 3 program (they will probably recommend a hearing eval anyway). The therapist would come to your home, evaluate her, and then if she qualifies for services, would work closely with you, the parents, in order to educate you and give you loads of ideas about what you can do with her at home during your normal routines. A common misconception is that early intervention services are free... they can be if you qualify for free services (like if you have Medicaid or have a low income) but payment is usually based on a sliding scale of your household income.
Good luck and enjoy this fun time in their lives!