[Gale slows, stopping, turning to Astarion and searching his face as much as himself for an answer. While he doesn't think his mother poses any threat whatsoever to Astarion, he does agree that it's odd she said nothing about his being a vampire. He'd suspected long ago that his mother would suss out his true nature almost immediately, but he'd assumed she'd say something about it, as well.

His nose wrinkles, his brow furrowing as he thinks aloud.]

My mother is not known for deception, not known at all for holding her tongue when she has something to say. That can only mean she had a reason to do just that, one she felt was good enough to keep from jeopardizing the night by voicing her suspicions. She's long wanted me to settle down, and is unfortunately justified in her doubt that I would ever bring home a suitable partner - and given she cares only about my happiness and relative safety, the bar is dreadfully low.

[And that he couldn't even bring home someone she thought made him — and would continue to make him — truly happy is a painful reminder of one of his greatest inadequacies. Still, it does lead him to a plausible reason as to why she wouldn't reveal Astarion's vampiric nature.]

If I had to guess, she didn't mention your being a vampire because she thinks there's a chance I am unaware, and she doesn't want to be the one to break the news.