1. You didn't ask for the money, it is falling into your lap. Therefore you don't need this money. Why not wait until you need it?
2. If what you build is great and is a success, that $25K will still be there, and you'll have a better idea of what to use it for.
3. If you fail miserably (You don't even have a product to sell yet), the loss of $25K will strain the family. Without it, this problem doesn't exist, and you move on, possibly with $25K to use for something else.
Promise them that they will be the first investor when you decide that you need capital, and get back to actually building your product.