Diana growing quickly due to royal blood. I don't thing so, as royals have shown no powers. Kelly will be a big question mark. Too many people smoking the witches hat to change, power changes, etc.)
Diana didn't start "showing out" until Adalind took the spirit of Frau Pech. You'll have to go back to Season 3. The gypsy Stefania told Adalind to remove the paste that formed in Frau Pech chest cavaty then applied the paste to her stomach (Adalind wasn't even showing (or looked pregnant) at this point. As the paste was absorbed into her skin, the paste briefly formed a skull before being fully absorbed into her skin.
Adalind knew something was wrong, so she went to the Medical Clinic to get a ultrasound The doctor then heard what sounded like two heartbeats, and Adalind asked if she was having twins, but the doctor said no and that it was probably just an echo.
The show never took a position on the double heart beats, but I believe the powerful Hexenbiest and the unborn Diana were becoming one. Frau Pech was a old and powerful Hexenbiest, and this is why Diana's powers are so strong (Adalind was human when she became pregnant by Sean Renard and if Diana had any powers it would have been 1/4 Zauberbiest, from her father (Mother: 100% Human at the time of conception; Father: 1/2 Human/Royal, 1/2 Zauberbiest)
Stefania Popescu, Queen of the Schwarzwald Gypsies (Human) and Elizabeth Lascelles, Sean Renard's Hexenbiest mother, were friends when they lived in Europe, but had to run away to Russia because the Royal Queen wanted Elizabeth and Sean dead. While Adalind is in Vienna we never see or hear of Elizabeth Lascelles, but it is my opinion that Elizabeth Lascelles had a hand in making Dianna more powerful, so Sean would one day be King of the House of Kronenburg. Adalind got her powers back after she buried Frau Pech's hand and feet in the field of flowers, yet Stefania Popescu continued to have Adalind do rituals. I think the rituals were more for Dianna.
We might not see Elizabeth Lascelles this year (Season 5), but she's pop up soon.
Yes, Dianna and Kelly will have problems, but it won't be anytime soon.