My niece is visiting right now. She is a darling baby, affectionate and smiling. Her parents are starting to be concerned at her small size and slow development. She has a few outward signs of Down syndrome (smallness, crease on one hand, slightly heavy eyelids) but has not been diagnosed as having Down Syndrome by a professional. She is eleven months old and can just now sit up, but not pull up to sitting or take weight on her legs. I'm concerned that she might need physical therapy that her parents are unaware of. Anyone have any ideas?

Eleven months and her doctor hasn't said anything?? I can't imagine a doctor not diagnosing a Down Syndrome baby in the first month, but not to notice a whole 11 months would be ridiculous.
Sometimes babies are slower to develop because their parents just don't know to stand their child on their legs, or to help them to learn to use their muscles more.
A single major crease on the palm may be one indication of DS but it doesn't necessarily confirm DS. You don't say how much she weighed when she was born but she may just be slow to start. Einstein was a slow starter.
Down Syndrome children are pretty easy to recognize. Usually the very small eyes that seem far back in their head and the flatness of the skull in back, although my Amy doesn't have that.
Many different things can cause developmental delays, including illness, mental and/or physical problems.
Have you discussed with her parents what the Doctor says when he sees her, and if they ever express their concern with her development? You might be surprised how many parents don't realize there could be a problem. Then again, there might not be, so you'd have to be tactful in how you bring it up, you know?
Good luck! Your niece has a wonderful Aunt to care as you do.
Peggy