AI just shipped a cake dummy for the first time to my sister, and when she got it there was ants in it! Has this ever happened to anyone before is there any way to prevent it?

I've never shipped cake dummies, but my suggestion would be to wrap each on in bubble wrap, very tightly

That way nothing can get inside

There's no telling what goes on in these shipping warehouses or trucks

Oh, gross, at least it wasn't a real cake, and you could have got worse critters I suppose!

I guess you could do the dummy, leave to dry well for a day or so, wrap well in plastic wrap, a few layers criss crossing each other, then bubble wrap, maybe foil for good measure too, then in a brand new cardboard box (just saying, in case you recycled an old packing box that may have had insect creepy crawly eggs in, they love to nest in cardboard) which has tape on the flaps and all the corner edges. 

I think you should give a heads up to the shipping company though and tell them what happened.