I know you must be very upset with your mum right now. But it does sound like she was trying to protect you. This news is obviously very upsetting to you, but it would have been so much worse when you were younger. I think your mum was placed in a very hard situation, and it sounds like she did the best she could.
And just because you have a different biological dad, doesn't make you any less a part of your family. Your "real" dad is the one who cared for you and raised you and loved you. Your brother and sister are still your real siblings. And your grandparents are still your grandparents. I can't imagine how hard it must be to try and comprehend all this new information, but I hope that when the dust settles, you can see that blood doesn't change who you are or who your family is.