I would not put the heifers back in with their cows until a couple weeks after the cow has her new calf.  It is absolutely critical the new calf get the colostrum right after birth.  The heifer may or may not try to suck and a good cow will generally not let last year's calf suck but the risk of losing the new calf's colostrum is too high to take a chance.

After a few weeks, the new calf has had its colostrum and cow and calf have settled into a routine. Then a good cow is more likely to give last year's calf a good kick than if heifer is but back in with her cow before or at calving time. Just my experience. Having limited pasture acres it is tempting to put them back together but emphasis needs to be on the initial health of the new calf.