A family, not poor, but no extra money to spare. Girl wants to play tennis. Has a sister, 7 years older than her, who was once a cheerleader. girl cannot afford tennis duds. Would one of her sister's cheerleaders skirts suffice to play tennis in without being an embarrassment?
Does she want to play on a school team, in an independent local league, or just for fun with friends?
School team would probably have a uniform.
For fun she could play wearing just about anything that allows sufficient freedom of movement.
ETA: For play in a non-school competitive tennis league, you might find this helpful
https://www.sportrx.com/blog/what-to-wear-on-a-tennis-court/
My -admittedly very limited- understanding of sports (clubs) that require a particular dress code, is that they hold a small collection of spare clothing from when members leave or outgrow (one way or the other) their current apparel. It also depends on the club; My local tennis club would happily allow you to wear a full suit of armour if it meant you turning up and paying court fees. Their only stipulation is 'attire that won't cause offence'. Which is a massive grey area and one presumes to have been deliberately worded so. If you are partaking in competitions, ie, representing the club, then stricter dress rules apply, and if you cannot afford the dress requirements, then discretionary funding is available from club funds depending on your ability. (Basically, if you are not expected to qualify or do well, you get nowt, chum. But if you are in with a good chance of gaining silverware, then here's a check- it's blank...)