I understand your dilemma as I have had clients choose combo's that I wonder about. On one wing my theory is the customer's always right. On the other it's hard for we quilter's to let the love flow into a creation which jars us.
Here's my solution. Make 4 of the blocks and show your sister the results with the blocks in a square. Then lay them out with a thin border of the fabric between them. Then lay them out alternating block and plain square of fabric.
Then you can say that you wanted to give her the option of layouts. If she asks your opinion then you can admit to worrying about a "too busy" pattern and explain the options. Every one of my clients has been pleased to be included. Good luck.