Take the full ride.
Your sister will be graduating from college before you…right? When she is no longer a student, any college that gives you need based aid…expect your costs to double when your sister is no longer an undergrad student at least half time.
The merit award to UMD is not need based and doesn’t count number of kids in college.
You will get that award all four years assuming you meet the GPA cut off.
Take the full ride and don’t look back.
If your sister happens to get some need based aid for the upcoming year, consider that a gift to your generous parents!