How do I get my girlfriend's parents to like me again.

I have been going out with a girl 6 years younger than me. Iam 28years old. Her parents use to be ok with me before they knew we had feelings for each other. When they found out it lead to total disapproval. Her parents even called a private investigator to check up on her to make sure she wasn’t seeing me and even got her x boyfriend to provide them with dirt on me which they threatened me with if I didn’t stay away her. What I am not so sure about is that despite the occasional ‘caught in the act’ scenarios by her sister and husband (who both hate my guts), after one or two weeks later she finds ways to convince them that we never ever went out. Thereafter my girlfriend turns into her sister’s best friend and starts planning her sister’s husband’s birthday party. This reoccurs every time we get caught. Just to add, her sister’s husband was my best friend for more than 10 years. Things just don’t add up. Is this relationship just for the sake of being naughty of having a boyfriend? I am really serious about marrying her. We speak about it like it’s just a stone throw away, but actually miles away and maybe even a fairy tale. Please advise.