I actually think the group did the right thing with Sissi.
I agree with what they did in not telling Sissy, what bugs me is Ulrich's personal reaction to it though. She had a good cause and yet he just calls her a "rotten traitor" even though he's not exactly the most moral person around either.
She could have just snuck out, and helped them herself.
But she woke up with Jim and her dad there and if she waited until they left it could be too late. She wanted to help as fast as possible. Not the smartest thing to do, but it was well-intentioned.