You'll need custody, unless she'll turn 18 soon? Just to be clear, she can't just 'go home every six months', she must live in Canada and visit the US. If it looks like she's trying to live in the US without the proper visa, she may well be denied entry and that would be far worse. But visiting regularly is fine, as long as she has plenty of ties to Canada and isn't trying to visit for too long/too often.
If her father is neglectful, is there anybody else she could live with in Canada maybe?
Best of luck.