You can try to apply for the visa, there is no rule that says your parents cannot bring their step child to visit you. Whether that will be approved is 50-50 but I guess it hurts nothing to try. If you are talking about insurance provided by your employer, it generally only covers your spouse and kids, I have never heard of any insurance by employer that can cover relatives. But this is a question that you should ask your insurance underwriter, if you have other kinds of insurance. Technically you can buy insurance for anyone.