how to get help with extra bills?

I do not have any legal expertise whatsoever, but it seems to me that your mother-in-law's money is being used for her if you're using it to cover her living expenses (a portion of your weekly grocery bill, a portion of your housing cost, a portion of your electric bill, etc).  Perhaps you can take that angle in talking to your family about supplementing your family's income with her money since you've taken a financial hit in having her move in with you.

Best of luck!    

Is your mother eligible for Medicaid? (Her eligibility would be based on her monthly income). If she is, there are innovative programs springing up in a number of state that she might be eligible for that would provide various kinds of support to her in your home at no cost to you. One Medicaid-funded model is the PACE program, which you can learn more about at  www.npaonline.org [npaonline.org]. Another good resource for low- or no-cost local resources is your local Area Agency on Aging, which you should be able to find in the yellow pages or through an Internet search.

You might want to take a look at the article Should You Move Your Parent Into Your Home? on the housing channel of Caring.com. The section on finances has some hard numbers that might help spark a much-needed reality for with your siblings. Here's an excerpt:

A recent study by the National Alliance for Caregiving (NAC) and Evercare found that caregivers spend on average about $5,500 a year out of pocket to care for an aging parent or relative. A smaller study by the NAC showed the numbers may be much higher: Those who lived with their parent spent almost $15,000 a year for care.

How do people afford it? The study found that most make sacrifices elsewhere:

Almost half cut back on vacations and leisure activities.
One-third dip into savings.
One-quarter cut back on groceries and spending for their own health or dental care to help pay for their aging parent's care.

You might also ask the visiting nurses for advice about helping your siblings understand the financial commitment you have taken on by caring for your mother. They've got lots of experience with families going through the same thing, and when they were taking care of my dad I really leaned on them for advice and wisdom as well as the medical care they gave my dad.