XANTHIPPE- I would send your sister a text message telling her you are concerned for her health and very sorry to hear she has had a heart attack- perhaps adding if there is anything you can do that you are there.
Here's the thing- your sister or her JW relatives might not appreciate your caring thoughts- but you will look back with NO REGRETS and feel better about yourself showing real human empathy and caring. Your conscience will be clear and you'll be able to sleep good at night- knowing YOU did the right thing- no matter what your JW family does.
How they respond to your message isn't the important thing- how you respond to your sister's heart attack by showing care IS the important thing as it shows them that you are mature enough to rise above the JW family differences. This is how I dealt with my older elder JW dad in his final 6 months who I was not that close to, just called to see how he was doing, no drama- just show a caring attitude. Don't let them entrap you into JW debates- it's not important when life and death issues are at stake. My 2 cents here, I'm sorry about your sister, hang in there, sincerely, Flipper