You are getting excellent advice here.
I appreciate that your parents want to see you become a successful professional. Perhaps they need to see beyond medicine and engineering as possibilities for you. If it’s any encouragement my son is attending a small liberal arts college (Knox) in the Fall, and one of their well-known graduates is Vir Das, the Indian comedian and movie star. 
He may have started in Economics/Finance, but one theater class is all it took to change his path. He majored in Economics AND Theater. Perhaps this may encourage you. https://www.knox.edu/news/commencement-2018/das-speech
- It would seem to me you that would want to talk to a college advisor soon, and see about not flunking, and then, perhaps, the possibility of transferring to the school of arts and sciences. Simply put, you won't have much to work with if you fail out of school. Is the coursework currently too hard, or is it you like Engineering so little that you cannot apply yourself to even do moderately well in it? You won't be a successful engineer if you HATE it.
- Think through the possibility of minoring in Public Policy or Environmental Studies which you can translate into a fruitful career should you go to Law School or go to work right after school.
- Do you have to major in Mechanical Engineering? Other disciplines like Chemical Engineering, Biomedical Engineering, Computer Engineering or Computer Science might be more interesting to you and can, in some cases, be more lucrative, provided you are interested in these fields and can handle the coursework.
- Environmental Engineering is not really policy oriented. It's usually a BS Degree with heavy STEM focus (calculus, statistics, chemistry, physics, organic chem) with some life sciences (Biology) and, perhaps, Geology/Hydrology thrown in. You end up working for municipal water systems, the government, consulting firms or even large companies to help with compliance re: water table damage, ecosystem maintenance, etc. These are, generally well-paying jobs and it's a broad field. It's not policy, but if you minored in poli sci/govt., you might then be able to get a job in your area of interest. Of course, you can always go to law school after. Might this be a way to compromise with you parents regarding Engineering and also get you into the policy end through a minor?
- It sounds like the ideal scenario for you is to Major in Environmental Studies (not Environmental Science) with double-major or minor in Poli Sci./Govt/Public Policy. Or vice-versa. You can then move on to law school and then practice environmental law. @privatebanker has given you some excellent advice. You might need to package this in light of a legal career I know medicine and engineering may be the ideals for your parents, but a legal career is also prestigious.
Best of luck to you.