Rabbits naturally pee-out excess calcium that their body does not need. If this type of urine is happening frequently, you can cut down on the amount of alfalfa pellets you are providing and/or cut out the alfalfa hay. My recommendation is cut out the hay, as it is good to get them used to eating non-alfalfa hay early on (hay is the most important part of their diet)! While the general recommendation is alfalfa pellets and alfalfa hay until 6mo old, I find many owners rarely wait until 6mo to stop, as they too notice the excessive milky urine.

On a side note, are your rabbits separated from each other, or do you have plans to separate? Rabbits, regardless if siblings, will try to mate with each other once hormones kick-in, so to prevent pregnancy, they need to be separated until they are spayed/neutered. Please note that the topic of intentional breeding is prohibited on this forum.

Milky urine or a chalky white residue that remains after the urine is dry is not necessarily worrisome. As Wick said, their bodies natural rid themselves of excess calcium. The real worry is if they have bladder sludge, which is literally the consistency of toothpaste. Frequent episodes of calcium in the urine is probably diet-related… which isn’t unexpected in young rabbits eating an alfalfa-based diet.