Like your sister, I had to start with patterns that appealed to me, and this didn't necessarily mean easy.. but unlike your sister, I started with cushions, so I could see the outcome quicker, and get the confidence to deal with a big quilt. After 4 years of quilting, I have just started quilting the biggest quilt I have done and that is still just over lap quilt size! Although I am just finishing a queen size quilt top, so I will soon graduate big time.
I used the on point approach to a lap quilt for the lounge room, where I didn't have enough scrap strips to make the quilt as large as I wanted, worked wonderfully. I googled the sizes for the inset triangles and managed to find good instructions.
Your sister has to love it and understand the hours she will have to invest.. if the joy of the process is what she is after, this will be awesome. But if the joy is the finished product, it may become a UFO.. maybe a good get out of jail free plan is in order, ie 4 blocks in, and decide whether it is a quilt or a bed topper! Either way, hopefully she will love the craft and enjoy every minute of it, like we all do.