{SOLVED} Mesh bra/top edges distorting when using sliders

Hi. Can’t check the file right now but in the linked tutorials the settings are not the best
Use nearest face interpolated for uv and weight transfer

You've got a few problems. One, your UV islands aren't in the correct place on the CAS map. Your UVs are in the bottoms and the shoes area. If you leave it that way then when you equip pants and shoes on your sim the pants and shoes will pick up the bra's texture. Move the islands to the tops section. Select your mesh, edit mode, press A to select everything. In the UV editor, press A to select everything. Image > Open Image and open the CAS map. If your UV islands become stretched vertically, press S, Y, 0.5. I don't think they'll become stretched, however.

The other problem is the topology. In the center strap, between the breasts, you have a vertical edge that does not go all the way around. Remove by selecting it and pressing X > dissolve edges. There are also useless edge loops on the straps that serve no purpose and are just adding to the polycount. Hold shift + Alt and select the unnecessary edge loops then X > dissolve edges.

It's not a good idea to have loose vertices. The edging around the hearts has numerous vertices but those vertices are connected to nothing. One by one, select the hearts (the large faces that cover the breasts) and press Ctrl + T to triangulate the mesh. This will connect the vertices on the edges.

You've got gaps in the straps where the vertices are unconnected. Hold shift and select two vertices then Alt + M to merge them. 

The straps on the sides of the hearts are clipping through. Move them so that the ends are inside the hearts.

Basically, you should always get in the habit of examining your mesh and cleaning up problem areas before weight painting, transferring the uv_1 and vertex painting. Look for things like faces squashed between other faces (you don't have any of that), loose vertices that should be dissolved or merged, clipping, things like that. Pan the camera around your mesh in edit mode and look for problems, and then go into object mode and solid shading and examine your mesh again. 

And don't transfer weights or the uv_1 to the body. They've already been done perfectly. Separate the bra from the body by selecting a few faces on the body then press Ctrl + L. The rest of the body will automatically be selected. Press P to separate it. Clean up the mesh then retransfer the uv_1. Ctrl + J to join the bra and the body later after you've done the weights, uv_1 and vertex painting.