I’m sorry about your loss.
For the new buns, buns need to be separated by week 12 as hormones can kick in and one bun can severely harm or kill the other, and buns are missexed so often that there are unwanted litters so you will need to separate them as soon as they come home until a month after they are spayed, you can do prebond it after the spays since their smell will change and they won’t smell the same as before.

Yes. They absolutely need to be separated. For a start, you said that they’re just ‘most likely’ both girls. Incorrect sexing of baby rabbits is very common, and this is how people end up with accidental litters. Often a young rabbit’s first litter will die as she doesn’t know how to care for them properly, and it’s also dangerous for very young rabbits to get pregnant.

There are stories where rabbits get on fine before being spayed, but there’s also plenty of stories where rabbits have vicious fights. There’s no way of knowing which they will be until they have a vicious fight, so the safest thing to do is to separate them before they fight, which will make it much easier to bond them later. You can’t just pick out the positive stories and hope for the best. If you spend any time on this forum, you’ll see one of the most common questions is ‘why are my sibling rabbits suddenly fighting when they used to be best friends?’ When their hormones kick in, they become completely different animals, and can cause very severe injuries to each other. It’s hard to separate them when they seem happy, but it is the responsible thing to do.

I strongly advise you separate them now, but keep them within a couple of foot of each other so they can still see each other. You can swap them between hutches every other day or so, so that they are used to each other’s scents and don’t get territorial.

It’s really hard to say. Some mature earlier than others and some mature later than others. I had one boy already mature when we brought him home at 8 weeks, and the other one matured 2 weeks later around 10 weeks. So it’s hard to pinpoint a specific time and say 12 weeks or 16 weeks because you just don’t know until the behavior is present.