My Cat keeps taking her T Shirt Off?

Perhaps an E collar would work better? I would call your vet and tell them that the tshirt is not working and they can recommend something different. Or you could go to a pet store like petsmart and purchase an E collar for her, they have nice ones, not just the plain plastic ones, that she might not be able to get off. You will need to do something as you wont be able to supervise her when you are sleeping and she will surely rip her stitches out further.