I need help with fixing the weight paint please

I created a shirt for a child sim, I did transfer weight and for the most part it looks pretty good, however I always have this issue with the "shoulder twist" where they aren't painted good enough and the shirt stretches badly under the arms. I really don't know how to fix it and I've looked at other people cc and seen that theirs looks perfectly flawless under the arms and I want to know how they do it! I've tried to manually paint under the arms but it feel impossible because it paints through the shirt and goes everywhere and distorts it. I must being doing it wrong or maybe there's an easier way? please help! thank you, I link photos below! -- Leah



[img src="" alt="and this is the issue " style="max-width:100%;"]