Hi, A.. Well, my son did not walk or stand independently until he was 18 months, even though he would do what your daughter is doing, walk or stand with support. How verbal is she? You might try playing games with her, enticing her to walk a few steps without support to come to you and get a favorite treat or toy. This can give her the confidence to do it on her own without any enticing.
Is she around other children her age who are walking? Sometimes toddlers do the "Monkey see, monkey do" thing when they are starting to socialize, and if she sees other toddlers walking without support, she may decide that big girls walk, so she's going to walk too.
Make sure that when she does stand up and then plop down on her bottom, you don't laugh or discourage her in any way. When she stands up, even if she's holding onto something, praise her for standing up, and even when she walks holding your hands, praise her for doing this. Try holding just one of her hands and walking with her -- she will get more practice at balancing, sort of like taking one training wheel off a kid's bike so he or she can get a better sense of what riding without training wheels is like.
You can also sit on the floor and get her into a standing position, then take away your hands but leave them close to her body, so that she can see your hands are close, but she's doing the work herself. This will give her more confidence, too.
I think she just needs some more practice in stages.
My son was a late sitter and crawler, too. He wasn't motivated to get up on all fours, to sit, etc. I started out by putting him into the position so that he could feel this new sensation, and then he got interested in trying to duplicate it. He had a hard time balancing on all fours -- he kept falling flat on his face because his elbows were straight, and then I showed him how to do it with elbows slightly bent. Before that, I showed him how to sit by getting him upright and propping himself up with his hands between his knees. Then I helped him to see what other sitting positions were like, and how to get from lying to sitting to crawling and back again.
My son also was more into fine motor than large motor. He became much more verbal & intellectual than physical or sports-oriented. How they handle the physical things as babies can be an indicator of what they're going to be like as older children, but we parents can still help them to enjoy physical things with a little encouragement and coaxing.
Peace,
Syl