Don't! i have been with my husband for 23 yrs and married for 21. he proposed in front of family from both sides. Love him but....totally and completely unromantic. And there is no chance for her to think it over or say no. i know you think its a done deal and of course she'll say yes but...truly giver her the chance, will ya?
You can comfortably propose to your girlfriend in front of her parents or family by first ensuring that her family is familiar with you before the proposal day. You can as well consider asking her parents for permission if they are old-fashioned so as not to see it as an act of disrespect. You should actually choose the right day, time and opportunity that will actually get your girlfriend and her family off guard.