Another pretty popular story line:

The protagonist is the sibling of Clary.

No need to come up with a special entry, just use the one from the show if you want. Jocelyn aka your mothers gets taken by Valentine because she has the cup, demons appear, the protagonist gets saved by the nephilim.

You can also turn it into a love drama by letting the protagonist fall in love with a downworlder. Not too sure if Joceyln and Valentine would be a fan of that.