According to amtrak.com, Back Bay station does not have checked luggage service, while South station does. I'm not sure whether Back Bay has "Red Cap" porter service, but I'm certain that South does.
When you get to Union Station in Washington, the train will be met by "Red Caps".
"Red Caps" work for tips, but they can be invaluable in negotiating your family through the crowds and onto the train, and making sure your luggage gets to the right place.
Also, I'm pretty sure that your ticket on Acela will give you a seat reservation (i.e., a specific location, not just a guarantee that you'll have a seat). This is not true on Amtrak's "Northeast Regional" or cross-country trains in a coach car, but I'm pretty sure it is on Acela.
Enjoy your trip.