Conform > Increase Size by 0.10 is usually the first thing I try out. I haven't tested if it makes a difference, but you should go to the Modifty (top menu bar) > Cloth Layer Settings and make the bra on a lower layer than the shirt—it's worth a try since you said it worked out without the bra previously.
Assuming that doesn't work, I would go ahead and use Edit Mesh on this one. If if were a jacket or dress, you would much rather edit Skin Weights before editing the mesh, but for a tank top I think you can skip that step...
Use Edit Mesh > Face, and turn on Soft Selection. Click on the part of the mesh you want to pull away and activate the World Move tool (Hotkey "W"). Medium values for Radius/Bias should work well. When you are done you, the places where you pulled the mesh will need to be gently smoothed down, (or simply deactivate or weaken the Soft Selection and pull them where they need to be).
CC3 & Daz Tricks | CC3 to Unity workflow
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