Yes, it is 90% chance that the heifer is a freemartin... She gets sold as a feeder... the bull is always good but we sell our steers as feeders. Just because she has a set of twins does not make her genetics "lacking" in any way. Over the years we have had no sets to one set to 4 sets one year... I had one cow that had 13 lactations.... over her lifetime.... one set of mixed twins and one set of heifers and one set of bulls... plus 10 times she had singles... and had 7 heifers and 3 bulls out of them. She was nearly 17 when she went down with a 5 mo bull calf on her and I put her down. I have kept many of her heifers and not one has had a set of twins...
I do not say there is not a genetic disposition to twins, but like in sheep or goats, nutrition plays a big part in when you are getting them ready for breeding. We call it "flushing" in the sheep... and it causes multiple eggs to be released which is very desirable in sheep and goats. Not so much in cattle, but last year we had some very good pasture in one place and both of the cows that had twins this year were at that pasture.. so same bull sired both sets as he was running with the cows there.
By the way, one dairy that I tested for over 30 years had quite a few sets of twins... more mixed sets than not. We kept track when 2 different "free martins" were coming in regular heats as he had kept them for heat detecting the "good heifers".... and they got bred when the farmer was away one week... Came up pregnant. He had always sold his freemartins before that. So, he started keeping them, and he wound up with nearly 40% of them being good and coming into the milking herd... So it is not always that they are not good. He did use some of them as "heat detectors" since there was no way they could breed the heifers, and often the ones that were not "good" were more active at riding the others when in heat.
That said, I do not keep the heifers that are twin to a bull because my luck they would be 99.99999% freemartins. I have no problem keeping heifers that are twin to another heifer.