Should you keep heifers out of heifers?

No. All offspring from first-calf heifers should be sold because they are just going to be too small to be any good as replacements. I'd wait until your first-calvers are older, like after they've had their second calf to consider keeping any replacements from them. You may have an opportunity to keep some heifers as replacements from second calvers, but you're better off keeping heifers from cows that have proven time and time again to produce good keeper-quality calves.

First-calvers are still growing when they have their second calf, and loose their baby teeth after this time. This growth and teeth-replacement period is quite stressful on both mom and baby, and as a result the calf isn't going to be gaining well because of these stressors. Besides, this first-calf heifer isn't a proven dam yet, so it is better to ship her offspring and give her a second (or third) chance to redeem herself and produce a better calf. By the time she has weaned her first and had her second, her adult teeth would've grown back and she would've finally reached maturity. Mature cows tend to give bigger, and better calves than first-calf heifers do.