Well she is pretty much a prisioner in Kings Landing, she can't do much outside of remaining courteous and taking insults without fighting back, otherwise she dies
Put Arya in her situation for example, Arya would most likely die in the first week of her capture she would defy the lannisters until they got through with her and killed her, at the same time if you put Sansa in Arya's situation she wouldn't have the will to pull through
Yeah! Be happy about her marriage. Then still go on saying how loyal she is to Joffrey. Then go on to say how 'happy' she is in King's Landing. Oh! The pinnacle.....how happy she was to marry her father's beheader. And even not speaking truthfully to Lady Tyrell even when she tokd her that this remains between the three of them. With the kind of situation she is....I don't really expect her to do much. I still remember how she only thought of her marriage with Joffrey and having his children, "and those prince and princesses will have red hair".
I have a different opinion on Aarya regarding her. She could've just escaped. There are people over there who'd have helped her.
I don't expect Sansa to do something magnificent or bebthe badass girl but atleast not be stupid. Even when people wanted to escort her, "But Joffrey"...the same guy who tried to rob her of her respect by telling his 'guard' to tear off her clother. Thank god for Tyrion. The same guy who ordered the beheading of her father's head.
Ned knew this was gonna happen and he did ordered them to go North. "But I want to marry Joffrey and have little prince and princesses with him" Aarya is no different here. She is guilty too. Both of them are responsible for their condition right now and I don't deny that. It seems like....the only thing Sansa has in her mind is marriage and bearing children.
On another note....I really really like Ygritte XD Jon Snow's girlfriend. I really wish they'd have stayed the whole time but alas

Why do all the good characters have to die?