I agree that Sona has right on Dev's money but she may not feel right to use it for her brother's and her business. Everybody has their own thought process about when and where they can use whose money. Imagine yourself in that house where you are always being targeted for smallest of things. Would you ever feel comfortable to use their 'family' money for your family purpose? Except for some emergency purpose, one would rather gather money from some other means for sake of avoiding drama!
And speaking of Sona, we know it from various incidents that she gives a damn about money. This is what makes her character beautiful and will give scope for further story developments and realizations.
Through this shade of Sona, maybe Neha can realize the power of love and determination over the power of money.