Umm, what? Anything that has gestation similar to a human is probably incapable of this. Could you imagine these fucking alien beans duckin' and dodgin' in utero to slaughter each other with nubby hands and fingers? Lmao this is some Morrowind concept art shit
It's not unheard of for one (human) twin to absorb the other in the womb. It's more along the lines of a cellular mixup, though, rather than a battle. And of course the mother even being aware much less sensitive to it is nonsense.