Usually they start sitting unsupported around 6-8 months. From about 5-6 months, you may try putting your LO in the tripod position and she may be able to balance on her own for her couple of seconds:) At the moment, just keep letting sit up with support for a few seconds each time and she’ll gradually get stronger and be able to maintain the position longer.
My boy able to sit up with support about 5 months. And before he turned 6 months, he was able to sit up without support. Dont worry mummy, they will do it on their own once they ate strong enough and is ready to do it.