When my sister was still in high school (she just graduated), I let her borrow my old ZTA bid day shirt from the year that I pledged. It was a big thing at her high school...to wear your older sister's sorority shirts. I guess they did it as a "bragging" thing; they got to brag that their sister was in a top sorority at one of the Florida universities.
Another girl from my chapter had a sister who went to the same high school as my sister. She let her younger sister wear her homecoming and bid day shirts. My best friend is a KD at UCF and her younger sister, who also attended the same high school as my sister, would wear KD shirts. I don't see the big deal in it as long as they are wearing bid day shirts or homecoming shirts and not block letter shirts. Plus, it's our biological sisters...not some random person that found the shirt at a flea market. We know that our sisters would not do anything disrespectful while wearing our letters. Plus, many of our sisters will go off to college and possibly become part of our sorority. My KD friend's sister is now a KD at Rollins. (My sister isn't rushing at UF though

).
Last year, I went to go pick up my sister from high school. I felt like I was back at UCF. There were a bunch of girls walking around with their older sister's KD, ZTA, Tri-Delta, or DG shirts on. I look at it as a positive: it's an ego boost to know your sister wants to borrow your shirts so she can brag to her friends that her big sister is in a certain sorority.