Call it sibling rivalry of the worst kind: no matter how competitive you get with your brother or sister, you’ve probably never taken it to shark levels and eaten them. Do sharks really do this? Oh yes they do! But...why though?
The short answer: who’s your daddy? Sharks -- especially sand tiger sharks -- tend to turn on each other in utero when they have different fathers. When mommy shark has been getting busy with lots of daddy sharks, she can sometimes start out with up to 12 embryos from various fathers. The bigger embryos then turn on their half-siblings and eat the smaller ones.
How do we know this? Dead females (mostly those accidentally caught in nets off the coast of South Africa) have been autopsied in various stages of pregnancy. Before five weeks of gestation, there are usually a lot of embryos with different DNA. By the end of the pregnancies only one or two fetuses remain, one in each of the mom’s two uterine chambers, usually with the same DNA. When the remaining fetuses inside the mother were also autopsied, their throats and bellies were found to contain their brothers and sisters. Yeesh.
Think of it as in-utero evolution, where the strongest survive by ganging up on the weakest. After they pick off the other embryos, many of the surviving sharks don’t stop there. During their nine month gestation, many baby sharks go after some of Mom’s eggs, meaning it will be another two years before she's able to conceive again. Immediately after birth, these baseball bat-sized baby sharks are large and fearless, ready to hunt and avoid becoming prey. They even swim away from their mothers immediately, lest she eat them. It’s safe to say that sand tiger shark family reunions do not end well.
In-utero cannibalism seems to be isolated to only a few species of sharks, while other sharks often give birth to litters of up to 10 babies. Blue sharks can have over 100 pups at a time while whale sharks can have 300. Not all sharks have live births, either. Some deposit egg sacks in rock crevices and take off. Others carry their eggs around inside them and live off the yolk sac (there is no placenta for nourishment). Sharks -- they're nothing like us!
TL; DR: Sand tiger sharks eat their siblings and they are not sorry. In this case there are definitely two of them, so beware of both left and right shark!