@furke,
I totally understand you follow your jyotish. And lets say he is right too that your sister aborted her child in previous life. Did he tell you the reason why she aborted though? I believe if he wanted to tell, probably he could like you said he is very "gyani". Who knows, she has aborted because the baby has fetal health problems, mom has health issues, or could a baby be from rape. Moreover, lets say your sister deliberately aborted her baby in her previous life then, why your brother in law is going through the same phase. And i strongly disagree that because your sister did something in her previous life, he does not have a kid.
Furke, I don't want to say your belief is wrong, you can follow what makes you happy. I just wanted you to be little sensitive as people are going through harsh time phase, and already are disheartened. If you cant say something nice, then you dont have to write anything. But i just wanted to say that whatever you wrote was very insensitive.