I am weary of a vet who recommended very young (4-6 week old) rabbits to have raw grass clippings– it is not recommended for rabbits under 6mo. The gut biome is very sensitive and introducing fresh food like that (which is the same category as veggies) is dangerous as the gut biome is not mature enough to break down that material effectively. It can cause a lot of GI distress and potentially lead to stasis or other GI concerns as a result.
Alfalfa hay (loose) and juvenile (alfalfa-based) pellets are the two parts of the diet that would provide the full range of required nutrients at this point. It is natural for rabbits to prefer pellets because they are designed to be very yummy and nutritous (as long as you’re providing good-quality pellets; do not feed muesli/mixed pellets).
The sleep-wake cycle of younger rabbits can be pretty dynamic, so it can be natural for them not to eat much overnight when the home environment activity dies down, then perk up and be more active w/ eating when the household is awake. As long as there are no extended (i.e., several hours) periods of not eating combined w/ not pooping, it is likely OK (in the absence of behavioral signs of discomfort).
Also, please note you will need to provide separately enclosures for these two rabbits in the coming weeks as they will become hormonal and, regardless of what sexes or relationship they have with each other, hormones will either facilitate fighting and/or mating between them, and you do not want either. Even if they’re siblings, that does not matter to rabbits– they do not acknowledge family units like humans.
The answers provided in this discussion are for general guideline purposes only. The information is not intended to diagnose or treat your pet. Seek the advice of your veterinarian or a qualified behaviorist.