The information at the links below relate to problems between dogs who live together. Please note: Even though the exact specifics of the problems written about are probably different than yours, we suggest you read the the information thoroughly anyway, you should find it helpful for your situation.
Dog Fights and Fear
See this article for more on resource guarding between dogs - it's the most common cause of dog/dog relationship problems! http://www.whole-dog-journal.com/issues/14_10/features/Resource-Guarding-Behavior-Modification_20368-1.html
This Q&A focuses on tensions brewing between 2 dogs during meal time, as well as a developing resource guarding problem between the dog and her human; learn more.
What to expect as your adult dog/s and your new puppy learn to live together:
Click http://tinyurl.com/jy3ml5e for an excellent article from Smart Dog University on helping your older dog live happily with a much younger housemate!
Worried about how your older, adult dog and your new puppy are getting along? This article Is This Okay? Typical Interactions between Puppies and Adult Dogs provides great information that will teach you how to identify what's normal, and what may be cause for concern, as they learn to live together. Also read https://suzanneclothier.com/article/stop-poking-grandma-whats-fair-older-dogs-puppies/ for more on this topic.
If you're thinking about adopting siblings/2 pups at once, or have already done it! The article at the link below discusses the problems associated with adopting siblings/2 pups at the same time. It's a 3 page article. Page 1 discusses the reasons you might want to reconsider if you're thinking about it; Pages 2 and 3 discuss how to do things properly if you've already brought the pups home, so both of them develop in a healthy way: http://www.whole-dog-journal.com/issues/13_1/features/Problems-Adopting-Two-Puppies-At-Once_16190-1.html?pg=3.
Read Training in a Multi-Dog Household for a good step-by-step plan.