It wasn’t like they tried to keep it a secret, they really didn’t but people just thought of them as good friends. They didn’t know why.
People tended to believe that Fugaku and Minato were good friends. They were, but they were also lovers. Sure both are married to Kushina and Mikoto respectively but those two were also dating.
The only two people that seemed to know were Kakashi and Itachi, Rin and Obito knew too, back when they were still alive.
Now their family was growing again.
Mikoto had just given birth to Sasuke and Kushina was still pregnant with Naruto.
Everything seemed to go as normal until the day Naruto was born. Something went wrong, horribly wrong. The Kyuubi was ripped out of Kushinas seal and let loose on the village.
In the end, both Kushina and Minato died protecting Naruto.
Mikoto and Fugaku tried to get Naruto, to have him live with them. They couldn’t since no one believed that they were anything aside from good friends to their late Hokage and his wife.
Both Fugaku and Mikoto broke a little inside that day. Not only did they lose Minato and Kushina, but they also lost Naruto, who was also their son. Naruto was about to grow up without the people that were left of his family. Itachi and Sasuke were about to grow up missing two parents and a brother.
They could only hope that Naruto and Sasuke would one day befriend each other and give Mikoto and Fugaku a chance to be there for him.
That sadly never came true. Itachi was forced to end the Uchiha clan, including his last two parents.
At least now Mikoto and Fugaku could be reunited with their loved ones in the afterlife.
Or so they thought.
Neither Kushina nor Minato was in the afterlife. And due to not having a sense of time, they didn’t know how long it would take to get there. Why weren’t they here in the first place?
After what seemed like years, neither Kushina nor Minato appeared. What was more surprising was that Itachi came before them. He told his parents everything that happened before and after they died. Itachi also mentioned that, even though he never expected it to happen, he fell in love. Shortly after he introduced someone who only died shortly before Itachi himself. A blond young man, his name was Deidara.
Sometime after Itachi, Kushina finally came. She told Mikoto and Fugaku why she only now appeared. Minato had sealed the rest of her and his own chakra into Naruto. She also told them everything she could see from within the seal. And that Minato might never join them, seeing as he was forever stuck in the Shinigamis belly.
For some reason, both Itachi and Deidara disappeared one day. Only to come back sometime later. They told them about the war what was going on.
All of them hoped Sasuke, Naruto and Kakashi would be fine and win.
They had nearly expected one of them to appear in the afterlife but none of them had expected Minato to finally join them.
He told them everything about the war and Obito.
They were incredibly proud of their sons/brothers. To know what they did.
They hoped they could reunite with them one day. Including the new members of their family, they had yet to meet.