You were like 3 years old when they were formed in 2005 and your sister were 9. Of course you haven't even heard about this band before they apparently came back. You heard of them but then again you were too young to understand about music genre anyway at that time. They broke-up on 2013 (you 11 y/o, your sister 17 y/o) which is a high rise for music from Maroon 5, Justin Timberlake, Bruno Mars, Katy Perry, Taylor Swift, Miley Cyrus, Selena Gomez, Psy, Ed Sheeran etc. So the probability of you listening to their music at that time is less than 1%.
Just like the band Love Händel, even old lady still love their music. So the answer to your question is "It is normal for your 23 year old sister to be obsessed with the Jonas Brothers". If she got the money for it, who cares what she do.