Don't worry abt it! Baby typically start to learn to sit at 6 or 7 months. You can find videos on how to help ur baby to sit up. It's ok if she still can't balance while sitting at this stage. Just prop up some pillows and let her get used to the sitting position and guide her to do the tripod sit. You can google on it. By 9 months if she still cant, check with PD but i'm sure she wld be able to by then!