I read more hurt in that line over Jane than amusement. Elizabeth's happiness is deeply tied up with her sister's. She's happy at the Netherfield Ball despite all her disappointments because Jane's happy, she even likes the Bingley sisters more when she sees them being nice to Jane at Netherfield. Elizabeth's thinking when it comes to people who hurt, or benefit, Jane is not objective. I believe that if by some weird circumstance Napolean had helped Jane across a dirty puddle Elizabeth would always have had a soft spot for him in her heart. :)