Why is Mom the only one who can tell Dad things related to his medical condition?

So, yesterday Mom got a call from one of the nurses in charge of Dad's case. He had bloodwork done earlier that day in preparation for his next chemo treatment which will take place later today. The results of the bloodwork showed that Dad's hemoglobin is too low and he will need another blood transfusion. Mom set the transfusion up for tomorrow so that Dad wouldn't have to have a 12 hour day at the hospital.

Dad was upstairs taking a nap when the call came in, so I asked Mom if I could be the one to tell him the news when he came down. Immediately, she said no, that SHE was going to be the one to do it and no one else! I would like to know why Mom feels she is the only one who is/can be involved in this cancer crisis. Or rather, why she feels that only she and my sister are or can be involved in this cancer crisis. Since this whole nightmare started, I have felt excluded. When I say that I've felt excluded, I don't mean that I haven't been updated on Dad's condition as the need arises, what I mean is that I've felt excluded when it comes to being allowed to help Mom out etc. Okay, so maybe I can't do the big things like helping Dad get dressed, but I CAN do things such as setting up his meds, getting him things to drink, reading to him, etc, but I am not being allowed to do so.  And it's no accident that I'm not being allowed to help out. That is really hurting me, especially since it's coming right on top of being the target for Dad's mood swings.