Her cutie mark is a red bloom encased by something that I thought is corn at first, but could later identify as wheat stalks when I stopped a few frames later and got a better look at her cutie mark.  
There is something blue under the red bloom, which seems to be drops of water (could also be a small bird or another, blue bloom, it’s hard to tell, but I go with water drops for now), like the red bloom is getting watered.  
So her cutie mark is a red bloom that is getting watered and that is encased by two wheat stalks. This is indicating that her special talent is growing wheat.  
The red bloom in the middle stands for blooming wheat flowers, the two wheat stalks for the fully-grown wheat that is ready for harvest. The water drops symbolize how the wheat flowers have to be watered for the wheat to grow.  
So, judging by her cutie mark, she is a wheat farmer. Which brings me to the connection between her and Plumberry.
 
Plumberry is standing on top of her, which indicates that she knows her and that they are pretty close to each other. But they don’t look alike each other, they look very different, so they are definitely not related. Which leaves two possibilities; this mare is either a friend of Plumberry’s family, and therefore Plumberry’s friend too, and hangs out with her sometimes because of that, or, she is Plumberry’s foalsitter.
 
Close to the mare Plumberry is standing on, is Berry Punch. She stands almost next to the mare. And we know that Berry Punch, like this new mare, is a farmer, a grape farmer. As such, it wouldn’t be unlikely that both of them are friends with each other. They are both farmers, so they met each other and became friends because of that. This connection between them and that they seemed to be together in this moment, indicate strongly that they are friends.  
Now, after discovering this connection between Berry Punch and this mare, I realized that Plumberry and Berry Punch actually look slightly similar. The color of Plumberry’s mane isn’t all too different from Berry Punch’s mane color and the same goes for their eye colors. And their coat has almost the same color.  
This similarity makes it likely they are related somehow. Berry Punch is probably not Plumberry’s mother, the look of them is too different for that, but she could be her aunt or maybe an older cousin.  
Which, when we take that into consideration, makes the connection between Plumberry and this new mare clear:
 
She is a friend of Plumberry’s family, because she is friends with Berry Punch and because Berry Punch and Plumberry are related. And as such a friend, she sometimes spends time with Plumberry, as trusted friend of her family.  
And from the looks of it, seeing that Berry Punch was very near the mare and Plumberry, the three of them spent the day out together, just hanging out with each other and spending some time together.
 
 
Conclusion: The new mare is a wheat farmer, as indicated by her cutie mark. She and Berry Punch are friends, because they’re both farmers and that connection brought them together as friends when they met each other.  
Plumberry is related to Berry Punch somehow, indicated by their similar looks, and part of her family. The new mare is friends with Plumberry and sometimes hangs out with her, because she is a trusted friend of Plumberry’s and Berry Punch’s family.  
And on this day, the new mare, Plumberry and Berry Punch were spending some time together, indicated by it that Berry Punch is very close to the mare and Plumberry and that she stands almost next to them.

Plumberry looks older here

@Frustration in Excelsis  
Thanks for telling me. :)

I sense some ‘sweet pics’ of her. :3

@EddictedBrony  
You can bring it up here, if you want.

I took one look at this pony watching the short and immediately thought of the name “Floral Fire”, only if it’s ok with others.