Dear BF,
I'm very sorry to read that your sister is again in the hospital.
I actually remember the last time she was so ill and what happened then.

I'm wishing her a very speedy recovery and will be thinking of her and of you. I'm also sorry that you weren't notified sooner but am hoping that you will be kept up to date on her condition in a more timely manner from now on. I can understand your frustration.

I remember how difficult it was on you the last time your sister was hospitalized so please take care of yourself. OK?