I would look at this in a broader way. Firstly, the gains in this market have mostly already been made, and there is even a risk of prices dropping. Therefore the possibility of making windfall gains (as your sister has already done) in the furure are slim.
The second issue is the one of buying in partnership with anyone else, even family members (or maybe particularly in the case of family members). How do you get out in the future if she wants to hang on, or vice versa. Are there other people involved -- has she a partner who might take a different view etc?
My own instinct, given the family complications and the slim margins available here, would be to avoid this one altogether and go somewhere else with your capital.