Please forgive me for this following comment but....(I sometimes put my foot in my mouth).
Poor Nikki! She was way to young to have a litter of her own at the age of 10 months. She was still a baby herself. Hopefully you will take her to the vet and have her spayed before she has another litter.

Anyway...Your problem sounds like Abby has now decided she will be the dominate female of the two and is showing dominance aggression. Basically she thinks she is the pack leader between the two dogs. Nikki doesn't understand fully the role change and has developed a fear. Perhaps their play actually turned into a unfriendly fight in which Nikki was the loser. While you can work to help Nikki overcome her fear so that she is not afraid of all dogs bigger than her, Abby's owners also need to work with her on her aggressiveness.

Let's concentrate on Nikki:
First start out by helping Nikki to overcome her fear. Never use punishment to try to get her to overcome her fear (which I'm sure you already don't do). Let me use an example: If she cowers around bigger dogs and you jerk on her leash or force her in some way to be around them, you have reinforced her belief that bad things happen when big dogs are around.

Instead if you can consistently make good things happen when big dogs are around, Nikki will start to relax around bigger dogs and be able to be around them. To accomplish this you need to desensitize Nikki to the fear that bad things happen when big dogs are around. Use a reward program. What ever she loves (treats, toys, games, etc..) use as a reward. Vary the reward from time to time too so it doesn't get stale or boring. Find a location or place that you know other big dogs will be. It may be a park or training class or where ever. Take her there frequently. Make sure that Nikki is far away enough from big dogs so she doesn't feel threatened but close enough for them to be visable. When Nikki notices the bigger dogs give her a verbal praise such as YES or GOOD GIRL or whatever you have used to date. Immediately give her the reward. Each time she glances at the bigger dogs give her a praise and then a reward. After she gets the idea that "I look at a big dog and get a praise and a treat" you can begin to gradually move closer to the dogs and treat her for calm behavior only. If you notice the tiniest sign of nervousness on Nikki's part then stop and calmly retreat her. Keep her away from bigger dogs for play just yet. If someone comes over to you with a bigger dog please explain you are training and you need them to stay back! Getting Nikki upset or fearful when a big dog comes around would be a major set back until she's ready. Go slowly in getting her near big dogs again.

Once you can get close enough that she does not show fear have the owner of the big dog perhaps throw her a treat or her reward. Only do this if she is still calm. Be sure to do this with owners of big dogs that you know and trust that do not have dominate agreesive dogs. Their dog will be only a leash length away! Eventually you will start to see signs that Nikki is not fearful around big dogs and she can maintain her friendship with bigger dogs and not be afraid of Hunter when he grows up!

Take it slow as it may take some time and don't let her back around Abby until Abby's parents are willing to work with her on being a dominant agressive pup. Some socialization and obedience classes for her might be what that takes. That is a whole different topic though. Your concern should be Nikki.

One final thought.... It is nice for littermates and parents to stay in contact after the pups are weaned but it is not essential. At this point Nikki looks at Abby as another dog and not necessarily as her daughter.

Best of Luck in working his out.