When a heifer/steer/bull, re-starts sucking, they can be VERY creative on getting a suck here or there.
The female that she is sucking can be brought into milk by the stimulation of sucking. That will/can ruin the "sucked" female. It #1 can cause mastitis - #2 will eliminate any colostrom being produced for the newborn.
If you are really wanting to keep the heifer, try the weaners on the nose plus put her with mature cows.
But, when it comes time that you remove the weaner - or - she loses it - will you know if she is "cheating"????
Not much worse than having a healthy newborn go downhill because Mom doesn't have the colostrom so desperately needed.
Remember, she can suck a bag dry in seconds.