How do I tell my 5 yr old he has 2 older sisters (19 & 16 yr old) that are not in our lives from my husband's previous marriage?

You might wait. Although i agree that your 5 year old may enjoy meeting the other siblings, he or she may not yet be old enough to understand the circumstances and why they are not part of your lives. If you are estranged from them, i would recommend waiting until your child is old enough to understand divorce and the issues associated with it when children are involved.
Your child will enjo. Your child will enjoy meeting the older step-sisters. Many children get along well with their step sisters/brothers and as they get older will provide support for each other. I encourage that you talk to your child about the older step sisters ang get your child to know them.