I think the original article is a little loosely written and confuses the terms "embryo" and "foetus". The developing organism does not become a foetus until it is twelve weeks old. And also (as in the OP) is it right to call the organism "the twin's foetus" rather than, say, "a foetus which could have become a twin"?
I recall reading somewhere that until about 1850 or so, the official line of the RC church was that the soul did not enter the foetus until about four months. This then changed to the current teaching that the soul enters at the moment of conception.
Clearly, there must have some reason for this implacable opponent of abortion to think in this way. Had the church not changed its teaching then, presumably, it would have no objection to early term abortion.
I admit that I do not like the idea of abortion but would uphold the right of any woman to undergo such a procedure should she feel it necessary. I think it more productive to consider an embryo to have the potential to develop into a human being rather than giving it "human rights". By the time the embryo has become a foetus the situation is more difficult. If it has reached the stage where it has a functioning sensory nervous system then abortion will almost certainly involve pain and suffering.
We should not forget that the most active abortionist of all is nature ...