Did God design our brains to enjoy sin?
No. Part of the answer is that the way free will works (it seems) is that we have the power to change our brain structure over a long period of time. Free will gives us the power to make certain things enjoyable and other things not enjoyable to our brains (here is a link to a good article on the science supporting this view, click 'show transcript').
So we have the power to change our brain structure against the way God intended our brains to function. An example of this might be someone who occasionally gives in to anger. They start getting angry more and more as they stop giving people the benefit of the doubt. Then they start assuming the worst about people. This process literally changes their brain structure, so the angry person changes their brain into one that 'thrives' on rage and anger.
But this is not the way God originally intended their brain to work.
OK, so why did God design us so that anyone would ever find it a good idea to change their brain structure in bad ways?
Our desire to do this is actually an unintended byproduct of something good. It's important that everyone has a sense of self-interest. If we had no sense of self-interest, then people would forget to eat, sleep, and would do things like walk off cliffs without any fear of death. Our sense of self-interest keeps us alive and enables us to perform very many sensible, everyday actions.
But there's not just 'good' self-interest, there's also 'bad' self-interest. We have various needs and wants (like food, security, shelter) and messing other people around will sometimes fulfill them better. Bad self-interest is e.g. murdering someone if it will make your life easier, or abusing someone to get them to do what you want.
God can't give us 'good' self-interest without making us aware that our life would be easier if we did 'bad' self-interest sometimes, and thus all sin.
For this reason we all change our brain structure to find 'bad' self-interest enjoyable to some extent - like pride, envy, unjustified anger, and so on. Sometimes, those things will fulfill our needs better than being honest. It wasn't in God's original plan for our brain structure but free will gives us the power to do it.
And God had to give us these needs and wants, because if we had no needs and wants then we would be self-contained 'social islands', which is not a better situation.
But isn't there evidence that personality can be influenced by our genes?
Yes, but there's a reason why God made it like this. To start off, there's an important difference between personality and character. Character is about good and evil, personality is about everyday (non-moral) choices. Personality can never be morally bad (when it's defined as choices not connected to moral issues), although it can be 'inconvenient' in some ways.
If God designs much of our personality, then God is not thereby encouraging or enabling us to sin. I think that God gives us certain personality traits to glorify His plan (like if you are an extrovert God may want you to meet a lot of people and encourage them in some way or something, so God designs you to be an extrovert). So there's really no problem if God has a hand in designing our personalities.
If God designed us to have flaws in our character then there is an issue, because that would make God the author of sin. But I don't think there's evidence for this view in the same way there's evidence that our personality is designed partly by God. Studies will show that personality has a genetic aspect, but it doesn't show that with an evil or good character.
Practically, these ideas can be encouraging - we are becoming more like our true selves when we reject the 'dark side' of our nature (Gen 1:27; Col 1:15). We are not betraying 'natural' desires to do wrong, actually we are becoming more like the kind-hearted people God intended us to be.