I think its good that you're visiting her! I understand why there may be some animosity between you and your ex-SIL, but what matters is that you're there for this woman while she is very ill. I think it speaks volumes that the nurses are thanking you so much for coming. I agree that its a good idea to hold off on visiting so that your ex-SIL can visit her mother, but after that, I think you should go back to see your ex-MIL. It sounds like she enjoys having you there.