You don't say definitely that you still want the house, but I think you do?
If you don't, then what mathepac said.
If you do, and the cost of repairs is simply higher than you expected, I'd ring the agent, say you've had it looked at*, and the costs are X-much more than you expected or could afford. Tell them you're still really interested in the house, but you would have to reduce the offer by X. The worst that can happen is the vendor says no, you lose the house, and it goes back on the market; on the other hand, the vendor may be prepared to do a deal.
*you don't say if the family member is in the building / surveying business or just someone whose opinion you trust: you'll have more credibility if you can say "Builder quoted..." or "Surveyor said...".