Hm littlek, but you could work already as a teacher while
preparing for your educators license.
Look here
http://www.doe.mass.edu/GEM/req.html
You don't need a master degree for being an elementary teacher and the 2+ years it will take you to finish your masters, you could work already as a teacher and pay into the teachers' retirement fund, in addition to get other benefits your sister probably won't pay you as a nanny.
Once you're established and a teacher with a decent
salary and paying into your retirement fund, than you
could enroll in a master program.
Financially, figure out what 2 years of continued work
for your sister without benefits would bring you versus
working as a teacher with salary and benfits and retirement
package.