Can we take legal action for a sex offender dating my sister?

If he's still on parole/probation, you can tell his parole/probation officer. To find out, look him up on the Michigan offender tracking on the Michigan State Police website. If not, you're left with the sex-offender registry laws which basically prohibit the offender from physically being in specific places. This would include her school, assuming she attends. Otherwise, he's pretty much free to go and do as he pleases as far as his offender status is concerned. Have your parents considered a change of custody? I'm assuming there is a custody order since your sister is still under 18. Otherwise, have you tried talking to her about it in a non-confrontational manner, as a sister? Or her friends? What do they think? She's more likely to listen to her sister or friends than she is her parents. The only other option I can think of is contacting child protective services. Some CPS offices might "screen" this out since she is the age of consent, but technically, she is still a minor with respect to child protection law, and the argument could be made that her mother has left her without "proper care or custody" if she's allowing her to run around with a sex offender and "do as she pleases." I wish I could help more, but you might be left with hoping that she comes to her senses. Good luck.

In short, no. The Sex Offender Registry Act was an effort to inform the public of potential sexual threats living near them, not to provide a cause of action to sue. However, your description of suggests there are some possibilities for recourse. While 16 is the age of consent in Michigan (this is different in different states), anyone over 18 years of age cannot have sexual relations with minors (under 18 years of age). If they do they may be subject to criminal charges. If this is the case you or your parents may want to contact the police where your sister is living. This may be complicated by the fact that the "victim" (your sister) will not likely cooperate with the investigation. Also, the facts you present suggest there is a history of involvement in the Family Court (such as a previous divorce). If that is the case and your sister is still a minor, her father may be able to use the fact that the mother is allowing her to date a sex offender as proper cause to change custody. If this is the direction your parents choose to go, they should contact an attorney to help file the motion. Best of luck.