If the outside of those petals are pink, it's a very pale pink, almost white. I would not suggest using pink that's recognizable as pink, on light blue. They would have virtually the same value, and the eye would struggle to differentiate between them.
Do you know about converting photos to grayscale in order to check the relative values? You could take a photo of soft pink fabrics on white, de-saturate until it's grayscale, and show your sister how it would look.
Would she be open to medium to dark pinks on white?