Me knowledgeable 
Like everyone says all these milestones are very variable and every cp child is different which is why it's almost impossible to predict the future achievements.
Definitely get THE book summer recommends it is amazing. here Sitting with tight limbs and low trunk tone is always problematic and will depend a lot on whether stretching exercises keep the tightness under control. Long sitting (legs straight) is difficult for most 10 m old babies and they prefer the curled legs which provides a more stable base. So don't insist on legs out straight yet. That comes later.
There are various ways of sitting which aid stability. Cross legged sitting or side sitting is easier as you DD gets older. Corner chairs are available for straight leg sitting and providing stability. I would concentrate very hard on strengthening the trunk. Lots of tummy time(yes the dreaded tummy time!) placing her on all fours and in the bunny position, putting weight on her arms and shoulders, pushing up from tummy. Support her at all times until she is stable as CP children love to face plant into hard objects! If you can strengthen the trunk and neck this will allow the arms and legs to be less rigid as at the moment they are partly compensating for the weak trunk. Not entirely the crappy spasticity is a problem in itself.
Try for a little bouncing with you holding her to aim for some feedback through the feet/knees/hips. Not excessive and not in a door or jumperoo bouncer, they encourage bad movements. Physios hate them 
I hope you are getting good advice and demonstrations from your physio? If not look into the Brainwave program. They were very good for us as they give you an individualised program and a top to toe assessment of your child rather that a bit here and there from all the different therapists.
An amazing number (probably the majority though I don't have any stats) of children with cP will learn to sit and walk so don't be disheartened. If you can talk to people in RL and meet other mums in the same situation. Isolation is the worse thing with disabilities. Scope have a befriending service though we've never used it.
Be like me, drink
and don't panic 