You are right that there is no emancipation in Wisconsin. That means that you can't legally make these choices for yourself. If you move out of your grandparent's house and go to live with your boyfriend's family, it is possible that your family will call the police or the Bureau of Milwaukee Child Welfare to have you returned to your grandparent's home.
There are legal options available, such as filing a CHIPS petition on your own behalf. A CHIPS petition would get the court involved to assist with problems you are having with your parents. Another option might be a guardianship petition filed by your boyfriend's family.
You would do that at the Children's Court Center on Watertown Plank Road. You could also try talking to a public defender at the center, as they may be able to assist you with this process.
Whether you seek legal action or not might depend on how close to 18 you are. If you just turned 17 recently, then the legal options may be more helpful. I would recommend that you talk to a local lawyer, and you should be able to get help from a public defender at the Children's Court Center.