My daughter has some problem (luckily minor) on her left foot since birth and she need to go to the physiotherapy of TM Hospital regularly.
When she was six month old, she still can't sit steadily on her own. So when we brought her to the physiotherapy, we 順便 asked the physiotherapist why so and she said it was because the back muscles of my daughter is not strong enough and she taught us some simple exercise to strengthen her back muscles. And it works! My daughter can sit steadily very soon.
The exercise is the adult to hold the baby's 兩邊胳肋底 with both hands (from the back side) and 上下"un"吓"un"吓, sometimes turn to left and sometimes turn to right. The baby will use his/her force to reinstate to the middle. All these will help him/her to exercise his/her back muscles.
Sorry that it's difficult to describe in words without photos. Hope you will understand.
Regarding the crawling, I attended a seminar before. The speaker said crawling is very important to the brain development. It's better not to let the baby learn standing & walking first, otherwise they will not go through the "crawling" stage. Do more exercise with the baby to strengthen his/her arm, shoulder and back muscles to help her crawl.