Perhaps speak to the breeder and find out more about the personality of the older pup, hopefully, with their dog's best interests at heart, they should be able to advise you on what set up would be best for her new home. The breeder will hopefully be willing for Darlagh and the older pup to spend time together to see how they interact with each other now - or even a week's trial to see how they get on?
No one can predict the future and how they will grow up and interact together, other COLers will be more experienced here, particularly those that have had to rehome one dog due to their inability to get along, although every situation will be different. I would try to find out as much as possible about the older pup - I would know straight away when meeting the pups and speaking with the breeder which option feels right. So go along this evening and find out more.
We have made the decision to get another dog, Blodwen will be 6 months when our new 8 week old boy will arrive, it's not an option everyone would go for but feels right for our family.