Illyramnaa Hi, welcome to the Spine forum!
It is difficult to explain well in writing, so I have created a simple example on this issue:
Basically, the solution for your issue is to devise the hip mesh. Although some vertices are a bit useless, the movement of the vertices of the hip mesh looks like the following animated GIF:
Add vertices with the idea of separating the areas you want to deform from the areas you do not want to deform. In this example, the areas you do not want to deform should be around the button and the zipper, so I added vertices around them and gave those vertices weights against the parent bone of the hip mesh so that they cannot be deformed.
I also placed some vertices near the seam line that spans the hip mesh and the front-leg mesh, and aligned that vertices and gave the same weights on both meshes:
This ensures that the seam line does not shift when the front-leg is animated. In fact, if I remove the vertices of the front-leg mesh, it will look shifted like this:
If you want to align the vertices of one mesh with the vertices of another mesh, it is useful to check the Wireframe option:
You can download the Spine project and PSD of this simple skeleton here:
I hope this will help you.