Bill and Charlie Weasley were at Grimmauld place with the rest of the Order and the 'Kids' were upstairs. The Order meeting had just finished and everyone was busy so they were asked to go tell the 'Kids' that they could come down now. Leah Lily Potter had arrived at Gimmauld place two weeks ago and so far it had gone smoothly. She was sharing a room with their little sister Ginny at the moment. Hermione had a room to herself for some reason and would just look at Leah and Ginny knowingly when asked why. Ron also had a room to himself while the twins roomed together. Like always.
The brothers went to Ron's room where they assumed Leah and Ginny would be. They were wrong. They heard murmuring then a squeal before a thump and looked into see Hermione had knocked Ron to the floor and was sitting on his stomach grinning at him. The older brothers looked at them suspiciously, they looked back innocently. This made the brothers more suspicious, everyone knew the couple had an on again off again romance. They probably shouldn't have been left alone together because at the moment they were on again. There was a piece of paper on the carpet between them and held a strange looking contraption next to it.
"A Pen" Hermione explained when she saw them looking curiously at it. Hermione rolled off Ron and gracefully stood up, making no move to help Ron who just kept laying there staring at them upside down, grinning.
"Well anyway you are allowed to go downstairs now" Bill said looking at them oddly, they brightened, making both older brothers laugh slightly. They knew it was unfair to not include the kids in what was happening even though the brothers wanted them to keep their innocence. The brothers turned and left to go get Leah and Ginny, when they were a little way down the hall they heard Hermione shriek slightly then laugh while saying "Ron, Ron please sto-" and dissolve into more laughter. The older brothers shared a smile, glad their younger brother was happy.
Continuing on to Leah and Ginny's room, they saw that the door was closed. In hindsight that should have been their first clue but no. The brothers frowned slightly, their mum had a rule that doors shouldn't be closed when others were in there with one of her children.
There was also a silencing charm on the door, which should have been their second clue, but again no. This was because the oldest Weasley brothers had seen both Leah and Ginny as baby's and didn't even think about what everyone would have already reached a conclusion about what was happening behind that locked door. So being the naive big brothers they were they decided to play a small prank on the girls in the room. Casting a silencing charm on each other Bill picked the lock. What hardly anyone in the Wizarding world knew was if you picked the lock a muggle way it bypassed all Charms and Wards placed on a Door. The brothers disillusioned each other and crept through the door, only to get the shock of their lives.