You're best option is to find a patch of similar 4-way stretch lycra and have it sewn and/or glued on the inside. This requires the right fabric glue and machine that does stretch stitching. Also, the tear should be closed and lined up as well as possible so it all lays flat. Odds are you'll need to go to a pro, and it'll never look perfect, but can be pretty passable.
If you don't do anything, the hole will continue to grow as the fabric, which is a knit, unravels at the edges.
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