I was actively looking for a summer internship. However, last week, I learned that I was accepted to an MS program that I must start in the Spring 2016 semester. After moving to the U.S., I have not visited my home country for the past 3.5 years. My plan was to complete the BS by Fall 2015, do an internship in Summer 2015, visit my homeland in Spring 2016, and start the MS in Fall 2016. If I cannot go visit my country in Spring 2016 then I want to go in Summer 2015. However, I have had job interviews for internships that summer. How should I professionally explain that I can no longer take the job? Will this be a bad reputation for my future career?
No, this will not be bad for your reputation. This may have caused difficulty if you were going to look for jobs after graduating in 2016, simply because you would lose some work experience, but since you are already accepted to grad school you should have nothing to worry about.
If you have any interviews scheduled, contact them now to let them know you are no longer interested and cancel the interview. It is up to you whether you give your reason or not; if you don't, they will most likely assume you got another job. In either case, they won't think any less of you. For the companies that you have already interviewed with, it would probably be polite to contact the ones you think might have actually hired you. However, it isn't necessarily a bad thing to wait until they reach out to you first.
So, don't lead these internship companies on, but don't stress about letting them down either. Celebrate the fact that you have been accepted into grad school and you get to visit home soon!