Chris is right Jags..... you have nothing to feel guilty about. You have your hands full with your Mom and your BIL can deal with this doctor's visit for his wife/your sister. I also think he wanted you to deal with it rather than him. He sees you as better equiped to ask the right questions and process the information.... but I bet he won't say that. Sometimes you just have to rely on your own gut feeling about what you can do, say no, and then refuse to feel guilty about it. You can't be all things to all people... I am learning that!!
Your sister's diagnosis is so very sad for you all but for the moment you have your Mom. There are so many that love the river de nile and just float along.... until they are hit between the eyes with a reality they can not ignore. Now that it has hit home I do hope your BIL becomes an expert but for now he is not telling you anything that you don't already know. He's still a newbie. Remember that. Hopefully he will take a few tips from you along the way since
Grab the other end of the towel.... I think we are all hanging on it to. We will keep each other afloat