my mother would be devastated if DS believed she felt less for him than her other grandchildren. Not that its likely as she adores the pants off him.
INterestingly my grandmother adopted her sisters child 55 years ago (father also unknown) and my granmother never had a great relationship with my Aunt and in fact they were estranged for 20 years. My grandfatehr however stayed in touch with her secretly all that time and adored her.
This difference isn't really down to the adoption but the fact that my grandmother is a rather cold, unempathetic woman. Despite that, I do care for her though I accept that she doesn;t (probably feel the same about the rest of us, except my dad which is whole other long story!). Her inability to bond normally with most of her family is I'm convinced down to an odd wiring in her brain. That I love her despite that shows I have a fairly normally capacity to bond whereas she doesn't so I don't stress too much about it.
I guess what I'm trying to say is that the "norm" is to bond with people that you spend your life with (whether they share your genes or not) though sadly of course it isn't inevitable that everyone will conform to that norm.
Mostly people who think that it feels different adopted vs not adopted don't have any experience of either adopting or being adopted. Their views are coloured by the intense love they have for their child and that they just can't imagine feeling that way about any other child. They just don't understand.
No-one has an issue with the idea of a life-long passionate devotion to a husband or wife who you didn't give birth to and no-one questions why siblings love each other despite the fact that they didn't give birth to each other and in fact could potentially share as many genes ("blood" is irrelevant its genes that count!) with cousins they see every 10 years and have little love for and nothing in common.
I don't dismiss the fact that adoption comes with many potential issues for the adoptee and their family but that doesn't make their family any less their family (IYSWIM).
Your situation scatty is the perfect example of why current adoption practice is to make sure children know virtually from birth about their adoption. Evidence shows that children who have always known (in fact who cant even remember when they were first told) handle the difficult issues they have to come to terms with much better than those who find out later.
SHe is still your grandmother, nothing can change that now, what you have very brutally had to deal with is that she isn't the perfect grandmother you thought she was. Some people are pretty rubbish - my Dad is a complete dickhead (excuse language) and has behaved far worse to me than your grandmother has to you. And sadly (unlike your sister) I have to accept that 50% of my DNA is his!
I agree that you should try to find someone independent to talk to about this - your GP may have a councellor attached to the practice you can talk to.