It sounds suspicious to me that she knew your rates up front & yet still complained they weren't the $50 her sister charges. Obviously there was some reason she didn't have her sister quilt it for her -- did her sister refuse because of the fabric? Does her sister do sub-par quilting? Does she take 6 months to finish the quilting? You can't worry about what other people may or may not charge every time someone complains. If you've done your research & know what the market will bear, having one person that's unhappy with the price is not a huge deal.
I would gently suggest that it's really not a good idea to tell people to take their quilts elsewhere. Stand by your rates & the quality of your work and let that speak for itself. Make sure to keep photos of any customer quilts -- but especially ones where the customer might complain so that if it becomes an issue, you can show evidence of the quality of your work. Be prepared to explain the rationale for your prices -- cost of thread, needles, batting, fabric, a percentage of durable/multi-use goods (machine & servicing, lighting, markers, etc), plus how many hours you put into piecing and quilting. If the market changes downward, you may have to adjust your prices, but that should be the only reason. If people feel like they can "negotiate" with you by insulting your work, that will become a slippery slope downward until you end up like some quilters that end up effectively paying clients to give them work since they don't even charge enough to cover costs.