If the calves are in their own sac, the heifer will be OK. It both calves are in ONE sac..the heifer will probably be a freemartin. There is a blood test that can be sent in that will tell if the heifer is a freemartin or not.
Cattle reproductive physiology is weird. (Actually, every species' reproductive physiology is weird. Changes in reproductive physiology are the engine that drives speciation.)
Anyway, one feature of bovine reproduction is that twin calves generally have a joining of the chorion; when that happens with a mixed-gender pair, exposure to the androgens of the male twin effectively sterilize the female calf. (The converse is not true.)
In other animals, non-identical twins chorions do not grown together, so the hormones don't cross over, or not to the same extent.
It sometimes happens that a twin dies in utero, and a heifer is born with no one suspecting that her reabsorbed male twin ever existed.
You can raise a freemartin for beef, and she will produce much like a steer of the same breed. (I have eaten steak from a sterile Holstein heifer, and it was quite delicious.) Also, they are in demand for immunology research.
We've had several mixed twin sets born over the years and have been lucky enough that the heifers have been fertile. But the percentages do say most won't be.
As stated above you can raise them for meat. A heifer actually makes better meat than a bull that is steered. We have raised ours on the theory that if they are a freemartin then we could butcher them.
I never knew of this "phenominon" until I adopted a day old buckling pygmy goat. His mother rejected him at birth, he was born with a twin sister. She gave him to me for free and I bought him milk replacer and he became my baby. A few months down the road, he was down and in excrutiating pain. After talking with my vet he asked "was he by any chance a twin?" I said yes, and he asked if the twin was a doe, again "yes"....and then I got the bad news that more than likely my little baby was born deformed interanally, and he explained that it happens in cows normally. Gabhar died a few days later from complications.
I'd rather him jsut have been infertile than die though anyday!
We've had quite a few freemartins purchased & brought to our farm w/out knowing they were sterile... let me tell ya, it isn't very nice to pay a heifer's price for a freemartin! They certainly do grow fast, though & deliver very nice meat.