How can I get over being angry?

My sister and I were adopted when I was at the age of two by my grandparents.
My biological family life was pretty horrible from what I was told, and my sister was sexually molested.
Our childhood was amazing as we were living in our new environment and my sister and I gained security through the love our new adopted parents showed us.
I went to college and graduated and moved away from the hometown I grew up in and my sister bought the house we grew up in. My parents lived next door and she helped cook and kept an eye out for them. Three years ago my mom passed away and that left a huge void in my dads heart.
July of last year he had back surgery and lost neuro function to his legs and bladder. He needed a caretaker, and my sister has a family now and couldn't take the extra responsibility, so I quit my job and moved back home.
There was a plan set in motion before the turmoil. They were working on fixing up his old home and selling it to get money to build him a home on their property.
As the new challenge reared its head, the selling of the house was taken off the table, because I just wanted my own space if I was going to be here and taking care of him.
My sister and I have had a great relationship, rough at times, because she has treated me like her kid due to situations when I was a baby and she was the one who took care of me.
Tension between her family and I have arisen because at first my dad needed 24 hour care and I could not get out and make money at a job, so my dad and I made an arrangement that he would pay my bills.
His level of care has decreased and his need for me has as well. There are still things that I take care of. Like taking him to doctors in our hometown and in a city 130miles away.
When his level of care was not as intense, him and I made another finincial agreement that was not as much. This was just fine for me and I got a part time job to supplement the money for my bills.
Over the last two months my sister as well as my dad have been concerned about money and I was told they are broke. So I have been paying all my bills to not make a stressful situation even more taxing.
Two days ago I took my dad to his eye doc in the city 130miles away. He was having eye surgery and it was an in depth procedure that was to last four or more hours. Due to his chronic breathing problem he coughs and the surgeon was not able to complete the surgery.
At the post opp the surgeon suggested that I ask his primary care doc for more meds to control his breathing problems and minimize the cough for the next time the procedure was to take place.
After we left the office, I suggested to my dad that he needs to be more on top of his breathing treatments when I am not at home to make sure he takes them. If he does not follow what the doctors have already asked him to do there is nothing further to be investigated because their plan of action is not being set forth.
The rest of the day he was being critical of every move I made. In my opinion he was backlashing and trying to gain a little more control in what he is told or asked to do.
This lead to a blowout when we got back home.
I blew up at him, and have noticed my fuse getting shorter as the days pass.
I am not a control freak and want him to do all that I ask, I am just the person who is hearing what the doctors are asking home to do and making sure his health improves through these plans of action.
Since his level of care is not as intense, for the last two months, his payment plan we discussed for me has diminished. That was fine, but I have not even received what we agreed upon for the last couple more baths because I have felt guilty to ask because he is saying he is broke. I have paid my bills, but yesterday I mentioned that I was hurt because he was not keeping his end of the bargain by paying what we had discussed.
It has never been about money, but there are only so many hours in a day to complete my work at my job and take care of him. I am still performing a service for him.

My sister threatened to kick me out last night. This was due to her saying I cannot handle this and my anger level is bringing dad down and he should be happy as a clam these last few years in husband golden age.

I am doing all I can as a person to be happy, not allow the circumstances bring me down, but I am at a point where I am disgusted by my actions and letting his comments rip into me so badly that I get angry... please help.