When the Ring Met Red Wine

I've been engaged for six months now, and everything was going great. Until last night, that is. We were at a nice restaurant with my fiancée's family when she excused herself to the bathroom. A few minutes later, I heard laughter from inside, but it wasn't just her laughing, or alone. When I went in to see what was happening, I found out one of her cousins had slipped into the stall behind her and... well, let's say there was wine spilled all over my fiancée’s dress in a rather inconvenient place. They said it was an accident, but come on! It felt like my trust was shattered in that moment. I know I should talk to her about this, but I’m scared of what she’ll say or do. Is there any way this could still work out? Should I confront the cousin too?