I've never heard a Lucas shortened to Luke actually, they all just seem to be Lucas. I have a Jessica which I never shorten to Jessie really but do quite like it. According to all the stats Jessica is usually a top 10ish kind of name but I don't know that many at all.
I know a Billy with a brother called Tommy and a sister called Madeleine (Maddie).
Other ideas:
Archie, Callum, Conor, Dylan, Ethan, Eddie, Freddie, Harvey, Harley, Jonty, Jamie, Jake, Joe, Liam, Nathan, Robbie, Ronnie, Riley, Sonny, Solly, Sidney, Vinnie, Zach
Annie, Amy, Casey, Carrie, Callie, Connie, Courtney, Chelsea, Dulcie, Effie, Edie, Elodie, Georgie, Halle, Honey, Hadley, Hayley, Hattie, Ivy, Josie, Jodie, Kitty, Kayley, Kelsey, Keeley, Kenzie, Lacey, Lindy, Marnie, Macey, Mercy, Melody, Naomi, Polly, Romy, Sadie, Susie, Sylvie, Tansy, Verity, Zoe