I am sorry for all you and your family is going through. I am pondering what Jim and Pam said, and I believe it is part of the truth. I also believe that we don't entirely choose if/how we mourn, but rather the relationship greatly dictates the mourning. I see it more as you are not choosing not to mourn her, as much as she did not share a relationship with you that her absence left something to be mourned. Would you mourn the splinter or the pain it caused you, after you removed it from your heel? You likely feel bad, because we are conditioned that we are "supposed" to mourn, because she was "family". Let go of the guilt. Your feeling are YOUR feelings; they are not wrong, and no person or social convention gets to dictate otherwise.