She's been accused of awful crimes including treason. She was above the age of criminal responsibility at the time those crimes were committed. She's an adult now.
Bring her home, lock her up (certainly not a case for bail IMHO and anyway not sure she'd be safe otherwise) and let her stand trial in a court, in this country in front of a jury and the public. The facts need to come out, including how it was she came to end up in Syria in the first place.
She was a child at the time and that needs to be taken into account.
She has to be a citizen of somewhere and regrettably, that seems to fall to us. Bangladesh don't want her and I don't blame them.
Something has to be done - its not practical for her to stay where she is forever and a day. Unfortunately she's the tip of a rather unpleasant iceburg. A lot of countries are going to have to take back the citizens who decided to join ISIS and put them to trial - either that or they face justice in Syria.
Personally I don't want to share a country with someone who can talk dispassionately about severed heads in bins but she's still a human being.
I reserve my sympathy for those who were slaughtered by ISIS and for their families and also for the children who were too young to have a choice about joining ISIS with their parents and suffered as a result. At 15 you have a choice - especially when that choice involves stealing your sister's passport, buying tickets and travelling across Europe to carry it out