My wife and I have lived in my parents house with them for the last 13 years, my dad passed away about 5 years ago, and then we took care of my mom, by paying all the bills, including the mortgage and taxes. Unfortunately my mom passes away about 1 year ago, and even though she said that she had put me on the title that never happened before she passed. I have continued to pay all the bills, but now my two sisters are trying to force my hand and say that I need to pay all the bills here plus pay them something each month. The deal was that if the house should ever be sold, then it was to be split 3 ways, but I have been the one doing all the work here and the upkeep, is there anything that I can do, so that they can back off?

Well what she wanted to happen and what she did was two different things, So unless there is a will, what happens is you have to put the house into probate and all of the estate will be divided amount the heirs
That will be your sisters, so basically and if this is like my state, you now own 1/3 of the house and each sister owns 1/3 and the sisters can force this into probate, in fact you can never own it, until you go into probate. So the longer you pay the mortgage off, the more money you are throwing away
Also note that each sister now also owes 1/3 of the mortgage if they want 1/3 of the house.

So you get the value of the house, and see what the loan is, and the difference is the equity in the house, you will owe each sister their 1/3 of the equity.

If they will agree to accept payments get this in writing, since they could force the house to be sold.