Hey, I’ll just give a little opinion here.
On dating…I think there’s some truth to that in if God tells you someone isn’t someone you should marry, you should move on. There’s no point in wasting anyone’s time if your end goal isn’t marriage. People date different ways and that’s okay. Some will date a long time, others will court which may lead to a quicker engagement, but inherently, there’s nothing wrong with dating for as long as you see necessary. The longer you date, the easier it may be to fall into temptation so there’s always that to be aware of.
On the one…I don’t believe in “the one.” I don’t think there’s exactly one person you have to find and marry in this world and if you don’t, you’ve messed it all up. God gives us free will in those choices and discernment in who we choose. I do believe that when you get married, your spouse then becomes “your one” because marriage is a covenant for life.
Different people believe different things on dating and relationships, but ultimately, it’s up to us to pray and read the Word, seeking how God desires us to treat our relationships. Your sister’s church probably has reasons and good intentions behind their beliefs, but I don’t believe that’s the only way to go about it.