In recent episodes, various characters have been thinking about, and expressing, how life in the Dixit household will change once Dev is married. Mami and Nikki have been the most vocal about this change, and their comments have weighed heavily on Ishwari's consciousness. She wants Dev to be happy but is terrified that his marriage will make her obsolete in his life. All the little tasks she so loves doing for him will pass on to his wife, and she will be left with no material, every day connection to her son. And her fears are potentially valid... if Dev were to marry Natasha.
I say this with Dev and Natasha's first conversation in mind. She confessed to him that despite her talent for business, her deepest desire was to be an ordinary housewife. I assume this means that after marriage she would prefer to quit working and focus on managing the household, allowing it to define her identity. This is a perfectly valid choice but I don't think the Dixit family is the ideal setting in which to take on that role. As a housewife, in a conventional marriage, Natasha would expect to take on the tasks that Ishwari does now, engendering insecurity in Ishwari (as already seen) and creating a conflict of interest for Dev as to whose domestic management he prefers. For Dev, Ishwari would always come first on principle, which would create a very unfair situation for Natasha without ever fully assuaging Ishwari's insecurities.
On the other hand we have Sona who brings something wholly new and unprecedented into Dev's life. She is a friend and confidant, and most importantly, someone who doesn't count on him being invincible, since she has her own identity and purpose outside of their relationship. His need for her has much more to do with the emotional support that allows him to let his guard down and be himself, and less to do with the day-to-day running of his life. Therefore, rather than an overlap, Ishwari and Sona's relationships with Dev complement each other. Sona is able to anticipate Dev's blind spots with regards to Ishwari -- allowing him to be more sensitive -- and whenever Dev and Sona have a conflict, it is often Ishwari who applies the balm of compassion that softens their anger.
If we dare take a peek into the future, we see two very different possibilities. Sona brings something to the Dixit family, simply by being herself, which Natasha cannot, being herself. Dev and the Dixit family are not ordinary, and Natasha's dream of being an ordinary housewife would not have a chance to flourish in their midst; she wouldn't really be able to enjoy her work because of its constant conflict with Ishwari's. Dev and Sona on the other hand have the potential to carve out a relationship that takes the complexity of the situation into account, a relationship that will exist harmoniously with Dev's devotion to his mother rather than at odds with it.