I am looking for a book series about a pair of twins from a heavy-gravity world who are acrobats on their planet and who can do amazing things when they come to earth's lesser gravity.
They also may be secret agents of some sort...?
Does the Family D'Alembert series by E. E. "Doc" Smith sound familiar? The first novella was published in 1976, and 9 more books followed.
Jules and Yvette D'Alembert are a brother and sister team of aerialists in the D'Alembert family Circus of the Empire and also work as agents in SOTE, "The Service of The Empire", the imperial intelligence agency.
The D'Alemberts are natives of the high gravity planet DesPlaines, giving them unusual strength, speed and coordination. They put this to good use by operating the "Circus of the Galaxy," a spectacular combination circus/mobile amusement park famous in every inhabited system, entertaining millions and rarely visiting the same planet twice in an average lifetime.
But the circus is also SOTE's best kept secret, known only to the Head, their designated successor, and the Emperor. Managed by the reigning Duke, who is absolute ruler of DesPlains and head of the D'Alembert family, the circus is a proving ground for the best agents available and can provide a cadre of highly skilled professionals in many fields, as needed. Furthermore security is absolute, because it is run completely by family members who talk only to each other - and the Head.
So when the Circus is summoned to Earth it is time for Jules and Yvette D'Alembert, brother and sister Imperial Stars, to leave their place in the spotlight under the big top to their successors, and become what they were always meant to be: the Empire's top secret agents.
As E.E. "Doc" Smith is one of my favourite authors, I can confirm this is Family D'Alembert.
Goldin wrote everything that's not in the first book. If it doesn't sit exactly right, Goldin wrote a completely unrelated series called Agents Of ISIS which is also ten books, with incredibly similar names (Getaway world is #4 in D'Alembert. Sanctuary Planet is #4 in the ISIS series), but he swears there's no connection.