Older dog vs. newer dog getting older

Question
Hi! I am a proud owner of two Labs. Babe, the blond, is the oldest; I actually found her at what I guess about a year old, and have had her for about 4 years now. I about 5 months ago, I got a black lab, Marlee, and she is just over 7 months old. My question is about Babe. She was concerned and acted jealous when I first got Marlee, which I knew was to be expected. But then it was like Marlee was the best thing to ever happen to Babe, because they constantly played with each other, wrestling and all sorts - I couldn't get them to stop playing. This was from when Marlee was about 3 months old (I got her at 2 months) to about a month/month and a half ago. Now Babe acts extremely annoyed with Marlee and snaps at her when Marlee tries to play with her. Babe never wants to play with Marlee, and she spends a lot of time moping around, giving me a lot of sad faces. I try to get them involved with each other, by throwing the ball for them, but it doesn't last long.
I just wanted to know if this is normal. I wonder if its that because Marlee is in her rebellious, wild phase, and not really little anymore, Babe is trying to let Marlee know that she is the dominant one. Marlee no longer backs down to Babe like she used to when she was really little. Is this behavior change normal for older dogs living with a growing puppy? I'm afraid that Babe is very depressed, and that it might take a toll on her health. I appreciate any advice you might have in this arena. Thank you!!

Answer
I would start with a good vet check up.  Babe may be slowed by a physical problem and Marlee may be seeing it as a chance to replace her as the dominant one.  Spaying them if not already will help.  so will better leadership on your part.  aggressiveness.  Avoiding having 2 of the same sex also helps.  One important thing is leadership on your part.  The key to most behavior problems is approaching things using the dog's natural instincts. Dogs see all the people and dogs in the household as a pack with each having their own rank in the pack and a top dog. Life is much easier if the 2 legged pack members outrank the 4 legged ones. You can learn to play the role of top dog by reading some books or going to a good obedience class. A good obedience class or book is about you being top dog, not about rewarding standard commands with a treat. Start at http://www.dogsbestfriend.com/ For more on being top dog, see http://www.dogbreedinfo.com./topdogrules.htm

If you need more help, post back with more details.  

In some cases, 2 mature females will never get along.