It's helpful to know how old your baby is. In general, babies should be able to sit without assistance around six months of age. It's okay to offer support for a baby younger than that in a device like a Bumbo chair or Exersaucer, but she should work on sitting without those crutches as well. If she has trouble sitting up by herself, she probably needs to work on her core muscles in her trunk to improve her stability. Try having her lie prone to work on lifting up her trunk. Put activities out of reach so she has to use her hands to get them (and use her trunk for support). Play "airplane" by having an adult lie down on the floor and lift your child up above you. She will need to support her trunk to stay in this position. And, play games with her by holding her trunk and lifting her up and down on the floor so she will plant her feet and support herself (that's a workout for you as well!).
All content here, including advice from doctors and other heatlth professionals, should be considered as opinion only. Always seek the direct advice of your own doctor in connection with any questions or issues you may have regarding your own health or the health of others.