If my opponent attacks with yellow ambessa and I use hostile takeover from hidden does her attack trigger before hostile takeover

Yes — her attack trigger is placed on the Chain first, but it resolves after Hostile Takeover.

Here's the order: when your opponent attacks with Ambessa, Respected and Feared, she gains the Attacker designation and her "When I attack" trigger immediately goes on the Chain (Rule 383.4.e). That trigger is what gives you the Reaction window to play Hostile Takeover from Hidden — so the trigger has already fired and its target has already been chosen at finalization before Hostile Takeover is even played.

Then, because the Chain resolves newest-first, Hostile Takeover resolves first (you steal Ambessa and ready her), and then her attack trigger resolves. Stealing her doesn't cancel it: the original owner keeps control of the trigger on the Chain, and it resolves with its original target and effects unchanged (FAQ #563, Rule 359.3.f.4). So if the target is still there and still has less Might than her current Might, it still gets killed — even though you now control her.