Edit: when we sold we were up to over 400 telephone lines.
Make sure to be as clear as possible of the risks and be clear to yourself first and to them when will you cut your loses if things go south.
Otherwise, try to be as explicit as possible regarding terms and your plan. Usually family gives money based on relationship, not contracts and business/financial benefit and as such what is broken if things go sour is not a contract or cutting a bad business deal but relationships. So, watch for that.
2. validate it
3. make mvp
4. ask them and show it
5. make sure not to lose all the money or they might hate you