Jon Snow has always wondered who his mother was on Game of Thrones, but currently Bran Stark is the only person who could feasibly give him an answer. Bran knows everything about everything thanks to his powers, and he witnessed baby Jon being handed over to Ned in a flashback to the day of Jon's birth. That means Bran also learned the big secret of Jon's parentage, namely that he's the child of Lyanna Stark and Rhaegar Targaryen. But bad timing meant that Jon wasn't at Winterfell to hear the news when Bran showed up, so will Bran tell Sansa about Jon Snow's parents on Game of Thrones before Jon returns? Fans are eager for the truth to finally get out.
As of now, it's hard to tell how this will play out, especially because Bran has been acting strangely ever since he turned up at Winterfell. Every Stark reunion has a lot riding on it in terms of fan investment; after watching them suffer while separated for so many seasons, everyone wanted to see the Starks rebuild their family. However, Bran and Sansa's reunion in "The Queen's Justice" was underwhelming to say the least. It seems like having all the knowledge in the world downloaded into his head Matrix-style deadened Bran's emotions to some extent. He didn't explain anything to Sansa — not what being the Three Eyed Raven meant, not what his powers were, not what happened to him. If he didn't tell her such basic information, then it's unlikely he'll tell her about Jon.
Bran definitely had the opportunity to tell Sansa about Jon's parents in Episode 3, even hinting that he had something to share with Jon, but he remained cryptic instead of giving up the information. It does make sense; it's a sensitive family secret, and Jon really should be the one to hear about it first. There's nothing Sansa could do with the knowledge except react to it or explain it to Jon in a letter, which wouldn't be the most exciting reveal. It makes sense to save the moment for when Bran and Jon are face-to-face again, so Jon can learn the truth in person.
Unfortunately, there's no way to know when that will happen. Jon is busy trying to make a good impression on the Mother of Dragons, which may delay his return to Winterfell for some time. If Bran doesn't want to resort to playing a game of telephone through Sansa, then he's just going to have to wait to tell Jon what he knows.