"My sister/brother is doing X. That is who she/he is. She/he does that."
(acceptance of the person for who they are, not who you would like them to be)
"I am not entitled to the sister/brother I think I *should* have, I am only entitled to the one I have." (acceptance that there is no "other" brother/sister, only the one you've got)
The first line above was given to me by my therapist at my last session when I was dealing with trying to accept someone for who they were despite the fact that I wanted to throttle them for being so blind to the effects of their actions on those around them.
The second was given to me a few years back by a different therapist for how to deal with not getting into sniping sessions with someone I had difficulty dealing with, but was forced to spend a fair amount of time with on a near-daily basis.
They helped. Lather, rinse, repeat.
That being said, I have found the former to be more conducive to a longer lasting sense of internal peace than the latter. The second statement is good for acute situations, where I won't be dealing with the person again for a long period of time. The first statement seems to help me accept the person for who they are, not only in the moment, but also as future interactions occur.