@Flávia Oliveira Yes, OCD will attack what you most value. (Having said that, it's wonderful, by the way, that you care for your family. These days there seems to be a lot less care within families, and that is so sad.)
So, what you are experiencing is classic OCD stuff. So let's talk about how you would treat this.
First, you have to change your relationship with your thoughts. It sounds like you feel like your thoughts about your sister are "dangerous", or at the very least that they have meaning and significance.
What you have to understand is that the thoughts in your head have no significance at all. The thoughts have no meaning at all!! They are like wisps of smoke 💨. They are just thoughts, and they don't matter.
The way to beat OCD is actually to ALLOW the bad thoughts to run freely through your head. Don't try to fight them off.
I know this sounds counterintuitive.
Most people with OCD spend most of their day trying to fight off the thoughts. But it is a paradigm shift that everyone with OCD needs to take. You need to realize that the thoughts mean nothing, and you need to allow them to run their course. If you start to fight the thoughts, you are actually training your brain that the thoughts are dangerous.
But allowing the thoughts to run freely--that's just the first step. The next step is to train yourself to resist all compulsions based on those thoughts.
The compulsions are all our efforts to try to "fix our lives" because of the damage we feel that the thoughts have done. We have to learn to do NOTHING in response to the thoughts. We have to go do the dishes, feed the dog, go on walks--and live our lives. In other words, we have to learn to do NOTHING to try to fix the thoughts.
So I'll describe what this looks like for me, and then let's talk about what it will look like for you.
I had bad thoughts in my head all day about God. And what I learned is that I needed to just let those thoughts run rampant in my head without trying to stop them or fight them off. The more I was afraid of these thoughts or tried to "fix" them, the more tangles I got myself into and the more debilitated I became in life.
I have learned that an OCD treatment, you DO NOT FIGHT OFF THOUGHTS. You must ALLOW them to run around in your head all that they want to.
Instead, your job with OCD is to prevent yourself from taking any action based on the thoughts (which will be always a compulsion).
Therefore, when I had a bad thought about God, I needed to learn to go on with my life instead of taking any action.
This is the point of ERP (which stands for Exposure and "Response Prevention). The Thoughts are actually defined as the "obsessions". And the "response prevention" part means that we don't take any action based on the thoughts.
So, in your case, you have to practice allowing the bad thoughts to run around freely in your head. And then you practice doing nothing about the thoughts. Just go about your life.
The feelings of guilt will come. There's nothing you can do about that. But you don't have to feed them. Just allow the feelings of guilt to come while you practice continuing to go about your life. In time, the guilt will fade. The point is that you were practicing living your life, despite the fact that the thoughts and the guilt are there.
That is how you beat the OCD. You are actually not trying to remove the thoughts from your life. And you're not exactly trying to remove the guilt either. You are training yourself to live your life as a fully functioning human being, despite the fact that you may still be suffering from the thoughts. You are training yourself to manage and live successfully.