I don't think that would have taken that much time for the VA to do things of that nature. Would it have made some of the tension between Hawke and the sibling away because they'd be more similar? Yes. But why is that a bad thing?
It's not a bad thing, it's just a different thing.
Also the fact that our sibling is only in 1/3 of the game again was it really that tight on time that it couldn't be done?
DAO had several factors more time, and was still "tight on time." Unfortunately, it's not just as simple as going in and adding a few more lines of dialogue here or there,
especially if the decision comes late in the development cycle.
Your suggestion would go a lot faster earlier in the development cycle, but would actually take a lot longer, and be more expensive, the closer it gets to the end of the development time. So how difficult it would be would depend in large part on when we decided "This isn't going to work."
Making decisions in software development is almost always faster and cheaper the earlier you decide to do something (same goes for things like fixing bugs and so forth - the earlier you find them, the less cost is incurred).
On top of the actual costs of doing the content itself, is also the opportunity cost. There's pretty much never any time when people aren't working on something. Any time spent on one thing is time not spent on something else. So again, the earlier we catch it, the lower the opportunity cost. The later, the higher the opportunity cost.