None of them are technically wrong. Shortest is easily the most common & natural. "Most short" means the same thing and is just a less efficient & natural way of saying it, so most people wouldn't use it, but it can be useful sometimes for adding extra emphasis to the "most" part. "Shorter" is a common casual shorthand style, especially popular in phrases like "the shorter of the two" even though "shortest" seems like the more (the most) gramatically precise option (see what I did there - I used "more" when I could have used "most" - it's the same kind of situation). If the context is already established where there's only 2 people in your family then the chance of using the word shorter is increased. Still, "shortest" is generally the cleanest, most universal choice.