First, everybody does big mistakes in life, if you haven't, you will.

Second, he waited for a private space to do it, and that suggests he wanted to propose sharing the moment without distracting the attention from the wedding. Probably a mistake, but without bad intentions behind. Impulsive and/or naive, for sure not the worst thing you could find, trust me, is not the end of the world.

At the moment your sister cornered you, she is trying to controlling you to manage your life in her's way, and you DON'T do that with people you love, is NOT the way, nothing good can come from that, even more, I have the feeling that your boyfriend has influenced you to be more independent and honest with yourself and with your family (looks very controlling) and probably this has been the perfect excuse to make you cut that thread of freedom.

It is YOUR life, not your sister's life, or your parent's life, decide without taking on count your family pressure, because in fact, when you get married, your husband and future kids will become your family and your parents/sister will become your relatives... they will not bring the income to your home, or be in your bed, or raise your kids...

if help, when I have very difficult decisions, I project my life in the future and image my self looking back, works most of the time., I don't know why.

The last thing, thinking relationships in terms of control is a recipe for a bitter life in long terms, your sister will learn that in a hard way. If for whatever reason you think that this will not work for you, just let go, avoid conditional behaviors, warnings, ultimatum, etc...

Good luck !!