Disclaimer: JK Rowling and Cassandra Clare are amazing and I don't own anything except the plot.
Author's Note: HP- This is set in a world where Voldemort was defeated for good the first time and none of the characters that died as a result of Voldemort coming back are dead. Also Lupin has been the DADA teacher since Voldemort was killed. Quirrel and Lockhart never were. TID- Mortmain doesn't exist and Tessa found the Institute soon after she came to London. None of the deaths that Mortmain caused happened except for Benedict Lightwood (Gideon and Gabriel live at the Institute). Jem isn't dying and he is not in love with Tessa; Will and Tessa are engaged as are Gideon and Sophie. Charlotte has had her baby.