Are you wanting to hear that its okay to leave him, or are you wanting permission to not do it?
I personally don't leave babies and toddlers. While the medical needs of several kids require that I make sure a competent, trained caregiver watches them and not just anyone, it has nothing to do with when I leave the kids to get away.
We just took our 17 monther across the country to pick up our 6 year old from fostercare. She's still nursing and will be for awhile. I don't leave her. The others are 5-12 and they were fine with their caregivers.
I let everyone tell me I had to leave my baby and have couple time when my oldest was 9 months. So, I was a good girl and I left her. And, I was absolutely miserable and hated every moment of being away from her. She cried the entire time I was gone and was hysterical and angry by the time I returned. I realized then that it doesn't matter what others think, but what feels right to me needs to be what I do. Dh realized that you don't get quality couple time if mom and baby are not ready to be seperated.
If you are ready to leave him and want to do so, he will be fine in the competent hands of your sister. But, if you want to know its okay to feel like you want to stay with him, there not one iota of wrong with NOT taking time away from your infant. CF or not, mothers and their babies are meant to be together. Its how we are biologically wired and there's nothing wrong with feeling like you aren't ready. If he were 10 and begging for you to go, I'd say you might consider whether you are being over protective.
You and your baby will know when you are ready to be seperated. Listen to what your instincts and your baby tell you. He won't want you so close forever, and you won't want to hold him so close forever. Do what you feel is right not what others tell you is right, whichever choice you make.