Hi,
I spoke with my sister two dyas ago,,she is healthy and reports are fine,,i ll keep u updated,,,,,,
If your sister did IVF then most likely 2 seperate eggs implanted and then she miscarried one. Like ZevasMom says, this is quite normal and there is no reason (related to this) for her not to be able to carry the other baby to term.
I think you mean that she miscarried one, not that one was aborted... yes, the same thing (kinda sorta) happened to me, you can be pregnant with twins, miscarry one, and go on to have a healthy baby...
To answer your other questions, you can have twins by one egg (the embryo splits creating identical twins), or by two eggs (two embryos which are fraternal twins).
i am sorry i have honestly never heard of this. i have heard of someone miscarrying one twin but neer a doctor aborting one. was there any health reasons or did your sister think that two would be difficult?