I don't think it was pressuring to skip his sisters wedding, but that snippet I posted is from Russo's article about Rossi's exit interview, so it's nigh indisputable that Guerin heavily implied that staying in Minnesota would be what's best for Rossi.
After that, maybe they didn't even know his sister was having a wedding, but Rossi wasn't going to try skipping his training for it because he wanted to please his management. I'm not saying it's malice on the part of Guerin or anyone else, but Rossi was probably worried (probably irrationally) that if he tried leaving again, it would hurt his chances to impress his boss, essentially.
Or maybe like Wabit said, he didn't really wanna go that badly either, and hockey really was his top priority.
Again, wasn't trying to imply malice on behalf of Guerin, but he's still a kid trying to do what he thinks is best for his career.