Hi
This is my answer of your question. The point you are putting is correct that to have a child is the decision of husband and wife. and from normal point of view it is the couple's decision and the rest of the family should be support system. But many times its is not treated as the couples decision but they are forced to take this decision because of various family pressers and I know may people who has gone throw this. In the case of Dev and sona. your question For Dev why Ishwari's happiness importance because this is the story of a boy who become a man at the age of may be 8. This is the story of a son for his mother is no longer a mother but the some times become his child and the child who is very demanding and who want that her wishes should be get fulfilled. Ishwari feels that she have given the extra love and care and good education to dev so for the entire life dev should do the repayment for this by fulfilling her ever dream and wish. And for this somewhere dev is also responsible because he fells its his responsibility. when at his home his mother does not consider that he can have his own wishes and thoughts and his own happiness how you expect other will understand that. For Dev to fulfill his mother's wish means he is making the repayment. not consciously but subconsciously and this is not after sona's entry in dev's wife as gf or wife but before that also because Ishu and Neha love to give the Dev a guilt trip. From the child hood dev used to fulfilled ishwari's every wish and give the the special treatment because one reason that he loves his mother and other that he knows that because ishwari's behavior rest his sister's are not going to treat her as the ideal mother. Neha was always having the problem and niki and riya found the mother in neha. and some for this may be dev was feeling himself responsible.(actually he is not).
Now coming to the happiness of dev and sona when you sacrifice your own happiness for other person he always expect the same thing from you. dev and sona always done the sacrificed for ishu so after marriage she constantly expect the same thing .because the person who does the
sacrifice silently people expect the same thing from them. That why Ishu was very happy that sona is going to be mother because of from her point of view ones again and dev and sona does the sacrifice their future plan for her. and she know they the only one who will do this things for her she can't expect any thing from neha.
And why dev is doing this because of the fear of losing sona this the the biggest fear of Dev and for sona her biggest fear is to face the same situation where Ishu is not happy wiith her and dev is choose to fores one of his relationships and the fear that dev wil ones agin choosing his mother over sona.
That s why ishu happiness is more important for them for the time being.
I hope at the end of the track both will learn their lessons.