Really sorry to hear about your sister.
I like Cadence and think it goes the best with Shelley.
I quite like Aurelia but don't think it works with Shelley at all.
Not keen on Constance but do like Connie. Bonnie I'm less keen on but it's quite sweet.
Naturally, I would put Shelley with other names popular in the 70s/80s/90s:
Amy, Beverley, Caroline, Claire, Courtney, Chelsea, Chantelle, Danielle, Emma, Fiona, Hayley, Jade, Joanne, Kimberly, Kayleigh, Kelly, Kerry, Lisa, Leanne, Laura, Michelle, Melissa, Nicola, Nicole, Natasha, Natalie, Paige, Ruth, Sarah, Samantha, Stephanie, Simone, Stacey, Shannon, Tracy, Tina, Tara, Valerie, Zoe
But, if you want something more modern/still popular, I also think it goes well with:
Amber, April, Alicia, Alexis, Amelie, Bethany, Chloe, Darcey, Eloise, Georgia, Holly, Jessica, Katie, Lauren, Lottie, Lois, Molly, Poppy, Penny, Ruby, Rosie, Sienna, Stella, Tilly