My sweet, sweet, Darling. I love you more than I can say. Having said that, I hate to tell you but some sleep training is coming at ya, my dear. I have put it off for far too long. I have babied you and rocked you and nursed you all night long. You're 10 going on 11 months old. You SHOULD be able to sleep longer than 4 or 5 hours without mommy.
Darling, it's not your fault (entirely) but I'm tired - extremely tired - more tired than I can EVER remember being before. So . . . I need you to sleep.
If you're sick or hurting, mommy will always be there for you and will be glad to see you at midnight, 4 am, and 6 am. Whatever. However, if your well and healthy, my darling - you need to sleep.
Your daddy is getting frustrated because he does not comprehend mommy logic (a.k.a. - it's faster to nurse her than to let her cry for hours - and "she might need me"). It's not his fault - he simply doesn't understand mommy and baby logic.
He's right. You're old enough (WAAAY older than your sis) to be sleeping better.
I love you more than anything my sweet-cheeked happy girl.