Andrew
I understand the sentiment - I know I trust my family implicitly and without reservation...but perhaps that is all the more reason to document it properly?
Also, as it is not an arm's length sale you should probably get a "friendly" agent's or agents' letter/s giving you an opinion on value for stamp duty purposes.
You can probably buy a contract from either the REIV or your newsagent...
but remember, a contract is a bit like a chainsaw - a useful tool, but mishandle it and you'll lose a limb!
Good luck
N.