actually fiona, in Islam it is a daughter's duty to care for her parents as much as a son's but especially in the case of there being no son. she has no responsibility towards her parents in-law, they are her 100% her husband's responsibility (and his brother's and sisters'). if she takes care of her in-laws it is out of love for her husband etc, but Islam does not require it of her.
exactly what i was gonna say
ur inlaws arent ur responsibility.. but if ur husband desires u to do things for them, u should do it out of respect for him as ur hubby's desires should be respected as well