Ripped mesh withsliders

Hi there Posh-Sims

You did not provide the link to the blender file (the link is missing from your post).

From the pictures I would advise you first (to minimize the issue) to delete the extra geometry that is not visible. That means the sim's body parts that are completely covered by the underwear. That way you limit where the sims body might become visible through distorted clothes.
Secondly this is due to a combination of uv_1 and weight paint that are slightly not adapted to the mesh. Due to transferring from a naked body for a mesh that sits above a naked body. That means the uv_1 is probably fine for most figure but will show distortions and wrongness when hitting the extremes of the slider.
As the mesh you created in MD was apparently not retopologized, the position of each vertices differs a lot to the ones from the body of the sim. that means that even with the same weight and equivalent uv_1 transfer they will simply not match exactly how the vertices from the body part move.

But basically to fix that: 1: remove the body parts that are not visible so they simply cannot go through your mesh. 2: try to fix your uv_1 to match better the body expansion when using the sliders.