I agree with NinaNina both that you sound like a very supportive and loving person, and that you should make sure that you are getting the right support yourself. Being a carer can be grinding, if I'm honest, even if you have all the love in the world to give, and making sure that you are on an even keel is the best and safest basis on which to be providing support to your DH.
On how people react to hearing that someone close to them is suffering with D (or any very serious illness for that matter) - I can only speak from my own experience, but as far as it goes people seem to fall into two categories of those who can cope with it, and people who can't (the news, I mean, not D itself).
I'm always surprised at how strongly people react. There doesn't seem to be any indicator for how any individual will react, it's something that seems to be quite deep seated. Some people can just accept it, and want to reassure by offering everything they have to give and then some. Others, even close family and friends, can be totally nonplussed, proactive but desperately unhelpful, or on occasion literally pack up and leave, which is truly distressing.
I think with the benefit of hindsight, my attitude is that everyone deserves/we all need consideration and allowances for their flaws - your SIL sounds like she has yet to develop an empathic maturity that comes with coping with the difficult hands that life can occasionally deal you. This is something for her to resolve, not you.
All very zen, I know. As I say - hindsight. And I think a little light handed management of those who aren't necessarily reacting helpfully but can't just be ignored is a very good thing, and nothing that you need to be embarrassed about. If your MIL is onside with this, that's great.
NinaNina, I'm sorry that you feel that your sons are struggling with your illness a little - perhaps this is just because you are still 'mum' in their eyes and therefore permanent and indestructible :). They may need some time to get used to the idea that you are not! But most people, even those who are very supportive, are not prepared for the long haul. It's not something you can prepare for I suppose - it's something that you need to experience and learn on the job. Remember to keep the lines of communication open and clear, and make sure that everyone has a forum to discuss their feelings in without politics or embarrassment.
My very warmest wishes to both of you and your families.