Why do you think Brangane gave them a love potion and not just water or something else?
To be honest there isn't much evidence in the opera to suggest that Brangane was a particularly bright girl. While she is the only one equipped with the knowledge of her deed, the revelation of which could bring about a peaceful resolution, she instead is capable of nothing more than wringing her hands, allowing Tristan and Isolde to be caught in Act II (on top of everything else, she keeps a terrible watch).
In fact it could be said that Brangane's keeping the potion secret is directly responsible for the needless deaths of Tristan, Isolde, Melot, Kurwenal, as well as Marke's heartbreak. We see in the end that Marke was in fact understanding after he knew about the potion, which Brangane only told him about after Tristan is mortally wounded and Kurwenal is sent into a triggerhappy frenzy.