Wow.
Your parents are being crazy putting the two of you in the position of 'negotiating' your share of the inheritance!
They should decide what's fair and just give you what they think that is, taking into account whatever they think are the pertinent issues.
This way isn't negotiation, it's two siblings in the position of putting each other on the spot and having to pit their own self interest against the other's, and the outcome is less likely to be a balanced and fair assessment and more likely to be the result of who is more inclined to dig their heels in / stamp their foot / not want to seem greedy etc etc.
I can't imagine a strategy more likely to damage your relationship and cause resentment!
Obviously inheritances can do that anyway but, that's usually if the deceased have blatantly favoured someone or some ruthless type spits the dummy out and drags everyone into a nasty fight... rarely if the parents try to be fair and the siblings accept their wishes without dispute.
I'd be inclined to talk to your parents and say that being asked to divide the spoils is unpleasant, and due to the emotive disparity in background and too much at stake it can't easily be a calm 'negotiation' and it's highly likely to damage the sibling relationship as it will be nearly impossible to get a decision that both feel is fair.
Tell them that the will should be the expression of their wishes and that is the only way anything is going to feel fair.