I dont think your being selfish, purhaps if your brother and sister in law didnt have anyone to look after their kids they might just be forced to take a little responsibility. I also think your sil may need some sort of pschological help, who in their right mids would do this. You could give them an ultmatum, get help and ill help you, but while they both are acting this way i think you have done the best thing. My brother and sil have just had a baby, couldnt afford one, and so both have to go straight back to work, they have asked me if i can look after the baby for 12 hours a day 3 times a week, i have refused, i dont mind having my nephew a few hours, but what they want turns out to be a full time unpaid nanny. Dont worry you are not being selfish!