Oh, so exciting! My Sammy is a nine-month-old Bernese, and he was such a sweet puppy, so I'm sure yours will be too! The main advice I can give you about having a puppy is to make sure to take it outside ALL THE TIME to go to the bathroom. I used to take Sammy out every hour or two when he was a pup, and he learned pretty quick to go to the bathroom outside. I also found it really helpful to crate train him, and he has always slept in his crate. Especially where you have two dogs, I think it will be really good for your new girl to be crate trained so she can have her own space away from Marley.
My guess is that your new pup will ADORE Marley. As soon as Sammy met my parents' older golden, he just started following her around everywhere. But of course since Marley is young too, he'll probably want to play with the pup, and he might be too rough. Judging by Sammy's activities, I'd say dogs his age tend to play pretty rough, so you'll want to keep a close eye on the two of them to make sure the pup doesn't get hurt. When Sammy was small, he had no sense of self-preservation whatsoever (still doesn't) so you kind of need to watch out for them, since they don't do it themselves!
Feel free to ask me any questions you may have about Berner pups! I can only go off my experience with the one dog, but maybe I can be helpful! They are a wonderful breed, and I'm sure you'll love her!