Oneplusone, I am so sorry for these circumstances that you are facing. I agree with Monkey on getting a professional counselor to help guide you through this.
My Dad had the bypass surgery done. It is a big operation and is serious so it is stressful for the family members.
I completely, completely understand your statement that you do not feel anything for your mom.
Your posts remind me of my past and help resolve a couple of pieces of my puzzle. I didn't cry much when either of my parents passed on. For my mother (when I was 18) I thought it was because I transfered my grief to my dad (Well, he'll be next to go-who knows when-could be tomorrow...). I was not aware of the truth of my relationship with my mother, that she did not love me and never provided emotional support or guidance through my life (rather the reverse with dismissiveness and ridicule). But anyway, to avoid that treatment, I went emotionally numb. I couldn't really grieve her because I was emotionally numb. I grieved my dad instead.
When he passed (I was 36) I had already grieved for him and didn't cry nearly as much as my two sisters. They probably thought I was cold, but I told them it was due to the fact that I had children and they didn't-so I had the comfort of continuation of generation-the circle of life sort of thing that is very comforting in such times.
But no, I think it all goes back to that I was raised in an emotional void and was emotionally numb. So I just didn't feel as much grief even for my dad (whom I was close to), kids or not-previous grieving or not: I was emotionally numb.
I had always thought (with some pride) that I was in control of my emotions, but the truth is that I dind't have amy emotions to control. Pitifully naive come to think of it. Just another piece of embarrassment from being brought up that way.
Sorry that was such an extensive tangent, Oneplusone. You are perhaps, too, emotionally numb towards your mom and her husband. So you won't be (expected to be) affected by what happens to them. I can feel from your posts that you are strong on this point and there is no doubt/guilt/question about it. (Bravo, by the way!)
However, your sisters. They do not understand the way you feel. In this stressful time, they may use you as a diversion to focus their stress, rather than focus on the truth of their mom being in a critical condition. If they pitch this at you, you could respond that all of this is about (and their focus should be on) mom and not you.
To preserve your relationship with your sisters, perhaps you could approach them as their friend-a supportive friend to get through a rough time. As a friend that had never met mom. So you as a friend, would be focused on your sisters, rather than mom. Be very kind and insist that they leave the subject of your feelings towards your parents alone now, the (their) focus should be on mom. There is probably a fine line, but you have made your boundary. Your focus of course won't be on mom, but on your sisters, and making sure they focus on her. With so much focus on her, who knows, maybe they will have an epithany!
The only other thing I can say right now is that time marches on and this is temporary. Time before the surgery, the day of the surgery, and time after the surgery. It will pass.
Got to go, sorry no time to edit .