Their love and support was conditional.

You're still framing this as if you're better than them. As if you're love is purer than theirs (getting a bit creepy, dude, she's your sister not your wife) and you're better for standing by her.

Sometimes the most supportive thing you can do is tough love. Sometimes the best thing you can do for a person is to say "I love you, but you are on a bad road and I cannot accept the path you've chosen".

Your sister did something very bad. Cheating is a gross violation of trust. She deeply hurt her husband. She risked his physical well being (potential STDs). What if hse'd gotten pregnant off her affair partner and pretended it was her husband's baby?

You may not know the full depth of how bad, your parents may not have told you everything that happened. The fact they're cutting out your sister, honestly I think there's more to this story than you realize. "Our daughter banged someone other than her husband" generally isn't enough to warrant ostracizing 2 people.