So long as they've considered the realities of marriage and taken time to receive appropriate counsel, no eloping isn't a sin.
I also don't buy this notion that adult children are beholden to their parents' blessings. But that's another conversation.
I was at my first duty station at Ft Carson CO. My wife was in her last semester of college. We had discussed marriage with both sets of parents before I left for basic training with no apparent objections.
My parents insisted on a summer wedding at their church with their pastor officiating. My folks were so poor the mice moved out but the demanded we get married under their terms. Momma had a list of almost 200 folks and the rationale why each of them should be invited.
Her parents insisted on a winter wedding so we wouldn't interrupt their vacation plans, at the church her father pastored, with him officiating. They were talking about footing the bill for a cake reception at the church hall. The in-laws had a list of almost 300 people if they only invited kin.
As time went on each side became more deeply entrenched that their way was the one that would prevail. In the middle of it nobody bothered to ask us what we expected or wanted.
On December 12 madre flew into Colorado Springs, spent the night in a motel and met me the next morning at the El Paso County clerk's office. We took the city bus to the 2d Brigade Chapel on base and were married by CPT John "the Baptist" Riggs. In a little more than a week it will be our 34th anniversary.
Parents don't always have God's will in mind when their kids decide to marry. Both madre and I have (had) godly parents. They provided wise counsel over the years on many subjects. In our case the idea that Helen's boy and Ray's daughter wanted to spend the rest of their lives together turn them into irrational people.
madre is the oldest of six kids. The younger five did not want to follow the
error of the eldest so they had the fancy weddings, blow out receptions and storybook honeymoons. They are all divorced.
I am the youngest of four following the others into the world of the wedded. I am the only one married to the original.
We meant what we said for better, for worse; richer or poorer; in sickness and in health 'til death do us part. We believed then as we do now that our marriage was for two hearts to become one to serve the Lord. I believe that being true to our vows is more honoring to a parent than acquiescing just to keep the peace.
madre and I renewed our vows on our 25th anniversary. With the parents watching, our children stood with us and my FIL got to say the words over us.