By Anya Leon
December 21, 2015 at 09:47 PM EST
Tibrina Hobson/Getty Images

This article originally appeared on PEOPLE.COM.

Charlotte is on her own campaign trail: To be the best big sister!

Chelsea Clinton is pregnant and expecting her second child with husband Marc Mezvinsky, she announced Monday on Twitter.

“Next summer, Charlotte is going to be a big sister! Feeling very blessed & grateful this holiday season,” she tweeted along with a photo of her 15-month-old daughter reading Big Sisters Are the Best.

In September, the mom-to-be said she was looking forward to having another baby.

“I think my love for Charlotte has just opened more space in my heart and my brain that I didn’t even know was there,” she told PEOPLE.

And while Clinton’s mom, Hillary Clinton, is busy seeking the Democratic presidential nomination for the upcoming election, the It’s Your World: Get Informed, Get Inspired & Get Going! author admitted the politician — and dad Bill Clinton! — are devoted babysitters to their first grandchild.

“Watching them do all of that with Charlotte is just a lot of fun … an unexpected joy in this process,” she previously told PEOPLE.

“They are so interested in everything about her — getting to know her and being part of her life and part of her daily route — so it’s not just checking in once in a while, but checking in every day to see how she’s doing.”

But while Clinton loves watching the bond between her baby girl and the new grandparents, she’s joked about finding her own parenting style.

“Er, she definitely gives me advice,” former First Daughter Clinton said in March.

“Some of it I take. And some of it I sort of smile and say, ‘Thank you’ for. I’m sure this is a common thing with new moms with their mothers!”

Clinton, 35, and Mezvinsky, an investment banker, were married in Rhinebeck, New York, in July 2010.