No one knows who their other parent is. All that anyone knows is that one day, the pressure of being a prince and the first male alicorn had been too much for Breaking Ground, and he ran away as a result.
Twilight and Flash were devastated, while Bright Star felt guilty since he felt like he wasn’t truly there for his big brother. No one could find him and for years, he was declared dead…at least until he returned and with three triplets in tow, who he explained were his children.
Any questions on who their other parent is, Breaking Ground refused to answer. But to Twilight and Flash, all that mattered was that their firstborn son was back and he was safe, and that they had new grandchildren to boot.
As for Bright Star, he has complicated feelings on the matter since his older brother’s disappearance meant that the pressure of being the heir to the Princess of Friendship was transferred to him. But at the same time, he’s happy that his brother’s back and that he could meet his nieces and nephew.
The triplets, on their end, are a bit overwhelmed to have so much scrutiny placed on them. But getting doted on by their family has helped them adjust to their new life living with their father in Canterlot.