Genetic need for a silky

Jamison was a binky boy until he was 3. Amy came into this world sucking her thumb and later needed a silky to go with it. Becca sucks her index finger and now too needs a silky. Today while Amy had hers as she was laying on the ground, Becca came over, grabbed a piece of it and started sucking her index finger. Can this be genetic?? Amy isn't always in the mood to be quite so generous so we drove to the fabric store and she helped me pick out material to make Becca her own.