Forget about being embarrassed. Your step mum and your sister will gladly help you. Being women they will know about menstrual cycles. This happens to all females. There is nothing to be embarrassed about.
They would, no doubt, feel very sad if they knew that you felt embarrassed to talk to them about these issues.
As you are 12, it may be awkward for you to use a tampon. Your step mum and sister may give you some good advice on how to help you to insert one. I am not sure if they do different sized tampons, so you may need to find out.
How long are you on holiday for? If you know how long your periods last, it may be wise just to give swimming a miss until your period has finished.
Best wishes
Talk to whoever you would be most comfortable with, either your stepmom or your sister, chances are one of them has tampons