Part of her had expected such a reaction. The Obi-Wan who had eventually chartered the Falcon was much older than the man standing in front of her now. This one was likely from at some point early on during his exile on Tatooine. Late enough that he felt comfortable using the name Ben, but not so late as to have had the harsh climate of the planet take its toll on his physical form yet.
Still, Jaina wouldn't pretend she wasn't a bit stung. She knew she looked a great deal like her father, but there was part of her mother there too. And her mother looked a great deal like her mother, whom Jaina knew Obi-Wan would recognize on sight. Still, he said he found it hard to believe, which means she needed some way to convince him. She just wasn't sure how.
Ultimately, she decided to simply let the truth of her statement make itself known in its own due course. You can't rush the inevitable, Jaina. She could practically hear her uncle's words from so long ago whispered in her ear and, unlike when she'd been a child, opted to listen to them.
"I'm sure it is," Jaina replied evenly, having managed to at least compose herself. "I wish there was something I could say to convince you, but I'm not certain what that would be. The things I know for fact, likely haven't happened in your time yet."
She paused, briefly, before adding gently, "Although, I think I should tell you that, eventually, the Empire is destroyed and in my time, Jedi are once again charged with protecting the galaxy."