After one of the most bitterly fought presidential elections of recent memory, where bipartisan unity of any kind has been hard to find, one bright spot has been the warm relations between the Bush and Obama families.

That bond was further strengthened Thursday when Jenna Bush Hager and sister Barbara wrote an open letter to first daughters Sasha and Malia, welcoming them as the newest members of the former first children's club.

Their words were published on Time's website.

They recalled the first meeting between the four girls at the White House in November 2008 and wandering the halls — as well as sliding down the banister of the solarium.

"Now you are about to join another rarified club, one of former First Children — a position you didn’t seek and one with no guidelines," they wrote. "But you have so much to look forward to. You will be writing the story of your lives, beyond the shadow of your famous parents, yet you will always carry with you the experiences of the past eight years."

The Bush twins, who were away at the University of Texas and Yale during their father's first term, advised the teenagers to "enjoy college. As most of the world knows, we did. And you won’t have the weight of the world on your young shoulders anymore. Explore your passions. Learn who you are. Make mistakes — you are allowed to."

PHOTOS: Barbara and Jenna Bush

But they also confessed that their time in the White House had taught them more than any of their college classes did. In fact, Barbara revealed that she had changed her college major and career path after accompanying their parents on a trip to Burma, where they saw the life-changing potential of antiretroviral drugs for the country's HIV-positive children.

Barbara now leads a public-health nonprofit, Global Health Corps, which she co-founded. It provides fellowships for young professionals in the U.S. and Africa aimed at improving health equity.

They also called on the Obama girls to remember the people who work in the White House residence and to stay in touch with them — even their omnipresent Secret Service protection details.

"They were part of growing up for us: there for first dates, first days and even an engagement and a honeymoon. We know it wasn’t always easy — the two of you and the two of us were teenagers trailed by men in backpacks — but they put their lives on hold for us."

Photos | Sasha and Malia Obama grow up before our eyes

They also looked forward to seeing what the Obama women do next. Malia graduated from high school at Sidwell Friends this spring and is taking a gap year before attending Harvard University while Sasha still has two more years of high school left to go, which is the primary reason her parents are staying in Washington.

"You have lived through the unbelievable pressure of the White House. You have listened to harsh criticism of your parents by people who had never even met them. You stood by as your precious parents were reduced to headlines. Your parents, who put you first and who not only showed you but gave you the world. As always, they will be rooting for you as you begin your next chapter. And so will we."

Read the full letter here.

Photos | President Obama's farewell speech in Chicago