why is ishwari against dev's marriage?

This is a very interesting topic. Let me try to write what I feel about this character. I love Ishwari in this show. She has such deep layers within her, each layer so entwined and complicated so much that it becomes difficult to distinguish them or separate them. Ishwari owes her life to Dev. Her life is Dev.

As @May1982 stated she has her own flaws. Everyone has. We all do. The major flaw that drives her is she her selfishness. Yes, she is. Not only her, there are a few others in the Dixit house that are core selfish.

- Dev is selfish. Didn't we see how Dev brought Sona into their life - it was due to the selfish reason where his main and only concern was to bring the best for his mom, let the rest of the world go to hell! Didn't he justify his act in his own way?

- Neha is because in her heart of hearts she knows that she liked Ranbir and wanted to take their relationship to the next level. But she did not go ahead because there were other materialistic reasons that she weighed over her emotion for Ranbir. For her, the thought that she might have to face those bad days due to the lack of financial security in Ranbir's household made her hold back her feelings. There she prioritized money over love, materialistic advantages over family and relationships. It is true that she suffered in her childhood and she never lived her childhood. She matured well before her age. That is a pain and void that cannot be corrected, filled, or relived. All she can do is fill the rest of her life with so much happiness and contentment such that the particular void is obscured in her life as much as possible. She probably will go through that transient phase soon. That is where Ranbir is going to play a major role in her life. That aside, what am trying to say is she is very selfish in that regard, isn't she? But that is very justified in a way from her POV.

- Ishwari selfish? Absolutely yes. Coming to Ishwariji, she is one strong woman and an aggressively successful mother. She lost her husband at a young age. She was left alone in this big bad world to fend for herself and her four young kids. She was so strong and brave that she wasted no time and sorted her life and position and made a clear roadmap right away. She had less education and worldly knowledge, but she was one intelligent woman and a fantastic mother. She took the reins in her hands and made the most calculative and optimal decisions with the resources she had on her hand. What did she have? Just her four children and their aspiring future. She had to invest well for the future of them and she had no choice but to invest in the best resource she had. Not to undermine others, but given the situation, she could only do that much justice so that it was a win-win for all the kids. She blindly favoured her first child. That was not to give an edge to that child's betterment alone but for the benefit of the entire family. She is one super business-minded woman and the most successful mother I would say. (Natasha must take classes from her) Over the years, while working with her son as a team, she emotionally invested more in him and became unrealistically dependent on him for all her emotional needs as the head of the family. She considered Dev the man of her house. She had to. She had to face the world and she needed his help. Even though he was a child, she could not help but expose him to the unpleasant things of real life at a very young age. She had to let him deal with lot of things that his age did not warrant, but she had no choice. All she could do was stand by him to support him from any physical damage. But somewhere in her heart she holds herself guilty and unfairly responsible for the emotional and psychic trauma that she put her son through for years. The damage that she saw happen and was party to it. This feeling of self-reproach is what making her feel very protective of her son. She wants him safe. And she thinks that she is the one who can take care of him, and anyone else might harm him. Over the years she has also become so reliant on him that she fears that she might lose her son, who according to her is her life itself. Without him, she has no life.

This has become so long and I better stop. I can go on about Ishwari. This equation between the son and mother and the freshness of Erica brought me to this show. I am eagerly waiting for this crux of the story to unfold.