I didn't read anyone else's response, but my 1st thought was that if your cousins didn't even bother coming to your 1st shower why would you be disappointed or even remotely expect them to throw you a 2nd shower/sprinkle? not to sound harsh, but i'm sure you know what i mean.
secondly, i think it is TOTALLY appropriate for you to throw your sister a shower. I've heard its not proper etiquette for Mom's to throw showers, but never heard of sisters. Frankly, i think its a dumb rule anyway and have been to LOADS of showers hosted by moms, siblings, both, etc.
third, can't help you on the timing. you will be tired and stressed from having hte baby, but this is a once-in-a-lifetime for your sister. she deserves a shower adn if you are the only one who might possibly throw it, than you should definatley do so. imagine how disapointted she'd be in no one threw her a shwoer!
goood luck.
Mom to DD 7/06, DS1 8/08 & DS2 9/11!