Thoughts? He needs to man up.
Taking the phone call out of earshot...so disrespectful.
You needed to hear him confront her.
A possible semi-solution: have him compile an email to his sister itemising the issues and send it in your presence.
(Dear sister, To reiterate our telephone discussion.....and to make sure you understand ...etc.) (of course no matter what he says neither of you can control her but this is for your benefit and a chance for him to show you his willingness to stick up for your needs)
He needs to learn that part of his role in R is not avoiding issues that need to be dealt with: doing the hard stuff because it's the right thing to do. Right for your healing and right for the M. It's all about strong boundaries and protecting them. He should be the first one walking over hot coals to protect those boundaries. He should be the first one to be offended by others who are not 'marriage friendly' and the first one to lay down the law and/or separate from them.
Perhaps you should ask him to do some reading on these subjects. It's a learning process for him too.
Your marriage and your R should ALWAYS come first.