[He pauses for a moment, pondering how many of his cards to put on the table. He wants some sort of insight, but he doesn't want to give his sister any power. He tries to give a casual tone throughout all of this.]

For starters, she's been displaying an unusual forgetfulness and has been uncharacteristically awkward around social convention. The last time she acted anywhere near this odd, it turned out she was carrying twins. [That's a way to address the problem without making it look like a Problem - to paint it as if she may just be pregnant again. It's a way for him to keep damage control on the situation.]