OP - YANBU - the people on here saying you might get nothing if the house is sold for care are spectacularly missing the point, the intention to favour your DB is there, even if it doesn't end up happening. it's very unfair on the rest of you, just because you're mum is legally entitled to be unfair, doesn't make it any less morally wrong. If you started an AIBU saying "AIBU to leave my estate in a way that means my favourite DC gets the use of it and gets to decide when the others get their money (if it at all)", noone would say that was reasonable (unless there was a reason your DB couldn't work himself).
Your mother is being unfair, the only good thing is that you know now before she dies, so many people only find out that their parent has been unfair to them after their parent dies, what tends to happen then is because the parent isn't there to confront/ask for an explaination, the injured parties tend to take out their frustration on the person who's inherited. Unfairness that's not been 'dealt with' and explained in life can tear apart families.
I actually think you are completely entitled to ask your mother why she's done this. Say that from where you are sitting, is very very unfair andyou'd like to know if there's a reason for it. She doesn't have to give you a reason, but quite frankly, you don't have to be civil and run round after her in her dotage if you don't want to.
I would ask her if she's thought through issues of inheritance tax, for example, if she's not expecting the money to be raised from the sale of the house, is she expecting you and your siblings to pay it out of your personal savings? Also, maintenance of the house - does the will specify who should keep the house in good repair? If not, has she thought that as you will own 1/4 of the house, you will have to pay out of your savings 1/4 of the inheritance tax, then pay for 1/4 of any repairs to the property/replacing boilers etc, on the understanding that you will get some profit from the house when it's sold, but it doesn't have t be sold until DB dies, which could be after you do. For your whole life, she's just effectively making you spend money on your DB, not giving you anything.
If she only wants her DS to benefit from her will she should have the courage of her convictions and leave everything just to him, at least that way it's not going to cost you anything and you'll all know where you stand.
But please, talk to her about this, listen if she's got a good reason she's not told you about before, or if she's thought through these issues herself. Long term, she won't have deliberately thought "let's see how I can make as much trouble between my DCs as I can" - give her an opportunity to explain and fix this if need be. Anger at funerals is far too common.