Is it advisable for me to continue caregiving with my dad? My dad and sister are narcissists & are treating me poorly. I suffer from PTSD.

Yes, give Dads care over to your sister. Believe me, they will find its not as easy as they think it is. Its now time to take care of yourself.

If your BIL and sister want to take on your dads full care, then by all means let them. Why would you continue to care for someone that abuses you and treats you poorly? Especially if you're suffering from PTSD. Do you not feel that you deserve better? Well I do, so step aside from caring for your dad, get on with your life, get some therapy for yourself, and let your sister and BIL care for your dad.
You matter too, and it's high time that you realize that. God bless you.

You do not give information on your dad. What are you doing in the way of caregiving?
If you physically and mentally can not care for him then let your sister and BIL take over the care.
You can help with things that do not involve direct caregiving if you can and if you want.
But the minute that you are treated poorly leave the situation.
One of the first "rules" of caregiving is that you have to take care of yourself. If that means walking away from a situation then you walk away.

From your profile: "I have a Narcissistic BIL & Sister that are making my life difficult in so many ways. They prefer he is in their care

I have discovered I have some serious health probs. I no longer believe I can care for Dad."

So why not let your sister and bil take over your father's care?