A letter from your employer would be useful for the first. For the second, if you've not yet had your BGO interview, that might be a good place to mention it.
As a general matter, lots of candidates have family members with health issues. I had one several years ago whose mother was dying of cancer (and died during the course of the application process). Thus, caring for a sick relative is not, in and of itself, likely to make much difference for your USNA application, especially at this late point in the process. What matters in terms of a USNA application is the impact this activity has on your life. IOW, if -- as a result of the need to care for your GF, you were unable to pursue ECAs or sports for a period of time or had to quit work, that would be an important fact. Here, it sounds like you're actually doing more (recent employment) -- not less -- since your GF came to live with you.
Otherwise, this is something you could simply email your RD about without any endorsement. However, as noted above, a sentence or two about the impact of this would be helpful. For example, you spend three hours a night doing X for your grandfather. Or, you've had to forego certain activities (sports games, etc.) to care for your GF. Or, what you've learned from doing this in terms of service to others.