You should not agree to lock yourself in to buying the house. There is no guarantee the house will be worth anywhere near that, and the bank might refuse to give you a mortgage. Why would you pay rent for 5 years then pay the purchase price, when you could just get a mortgage now and repay it for the next 5 years?
Rent to buy is basically an option. You agree to rent for a certain period and then have the option to buy the house. If house prices fall significantly you simply walk away. This is the risk - because you paid rent for 5 years and have nothing to show for it.
However, the seller MUST sell you the house if you want it. In that respect, a rent to buy agreement would work out in your favour given the term involved. If the house went up €50k over the 5 years, you buy. If it doesn't, you can make a lower offer. The problem is you are dealing with family here. If you don't buy will your family member fell hard done by?