I am looking for a spot to send my mom. She is 50 years old. I will send her along with my sister.
She loved Wash DC because of all the historic sites. I need another place like that in the US.
Any suggestions? I was thinking Philly?
What do you think?

Take it from another 50 year old Mom, send her to San Francisco. It is an exceptionally beautiful city with a lot to see. This site has tons of recommendations on what to do, where to stay and eat, etc.

You might consider Charleston, SC with a side trip to Savannah, GA if time permits (2 hours away). Both have much to see of historic value, there are nearby beaches, and excellent restaurants.

Boston has the historical sites and sights to offer. Most of them are in a relatively small area. Driving in Boston is difficult and parking, when you can find it, is expensive. So a hotel downtown is almost a requirement.
If you Mom is interested in American history, there are many "package" day tours than can be reserved.
Another place with good motel availability is Williamsburg.

I would say it depends on the time of year and also how much you want to spend on airfare. The above suggestions are all great. I would probably not suggest Boston in the dead of winter or Charleston in the summer. But they are both wonderful cities. Also New York City has just about everything from history to art so it's always a good choice(also watch the weather). Chicago is a good choice(again not in mid-winter) if she likes shopping and art(the Art Institute is one of the world's finest museums). If history is her thing then Charleston-Savannah, Williamsburg or Boston would be my top picks. Another good New England choice might be Newport RI.

Here's another 50 year old mom who would agree with San Francisco (for interesting places to see, shopping and dining), but also Niagara-on-the-Lake (just north of Niagara Falls), NYC, and my top pick - MONTREAL! But, if the emphasis is on historical sights - how about Boston?

As a Virginian I vote for Williamsburg. There are several ticket options available through Colonial Williamsburg, you can get combination admissions to museums / visitor centers in Jamestown, Yorktown, Carter's Grove, and several nearby plantations within a scenic 45 minute drive. Busch Gardens Williamsburg also has lots of shows & attractions enjoyed by all ages, it will open up again in April I think.

Saint Augustine, Florida - historic sites, warm, beaches.