Hi Mafe,
One thing I've learned about being a photographer is that more often than not you spend you time looking for lines. Think about it; when you point your camera in any direction, you are in effect pointing a a laser line towards the horizon, and we are going to see anything along that line. Your job, is to find the lines that provide the best background for your foreground.
In this instance, the line you have chosen has resulted in this very bright, overexposed background. So, what would be the easiest way of getting rid of this background? Simple answer: use the trunk of this foreground tree as a background instead. All you needed to do was move yourself a couple of metres to the left and that bright background would be replaced by the solid trunk. As another option though, I would have probably gone looking for some trees that were a little closer together.
There are another couple of issues that are worth addressing in this image though. One is the posing; I probably would have asked your mother to sit up a little and lean forward slightly. This would be more flattering as it lengthens the body. I would then have had your dad move the slightest amount to the right of the frame and take his hand off her leg, only because it makes that part of the picture look slightly awkward. He could place his hand behind her for the sake of creating a diagonal, or place it on her shoulder.
Finally, there is a slight green cast on your parents' faces. Ideally I would select this part of the picture and add in a slight amount of magenta to correct for this colouration.
I hope you get to have many more walks in the gardens with your parents; they look like patient models to practice on.
Cheers,
Anthony