I'm going to take a slightly different view on this, and that is that it is your mother's estate and it is her will so she can and clearly will do what she chooses with that.
I think we'd all like to think that our parents and loved ones would make a will with the intention of dividing it up equally. However, does the concept of "fairness" really come into play here?
For example, if your DM believes (and it seems that she does) that your DH is earning substantially more than your siblings and thus, as a household, your household income is more than that of your siblings-is it possible your DM feels that in order to be "fair" she should leave more to your siblings as they may "need" it more?
This is a horrible situation and I think your feelings are absolutely understandable. Many families argue over the Wills. I'm not sure how you go about making peace with it-one thing I would say is that it isn't set it stone and it can change.
Speak with her openly and honestly about how it has made you feel-don't threaten to step back from her life or from caring for her-that just makes you look bad and like your love is contingent on how much money you get from her-and ask for her to explain her reasons for her decision.
Be open to hearing what she has to say. It may well be something that you simply have to accept and decide what you do with your relationship from there. Speak with your siblings and see what they have to say for themselves on it. They may well share your view and be happy to speak with your DM about this.
I've seen rows about Wills play out in my own family-over similar issues-and they're not nice or enjoyable for anyone involved. I'm still a big believer that what someone puts in their Will is their choice and they're entitled to have their wishes fulfilled. Whether or not people agree with the decisions and the reasons behind it.