I can understand how widows and widowers sometimes move on with their spouses siblings or friends. It almost happened to me.
My best friend and I married a couple of men that were best friends and lived on neighboring farms running the farms and raising our children together as one large family. After my husband and best friend passed away, her husband thought it would be a good idea if we married and continued running the farms and raising our children together. Our teenaged children thought it was a good idea too. I didn’t. Although he had no trouble transitioning from a brotherly role to a husbandly role, I could not stop thinking of him as just a brother or good friend.
I ended up selling my farm and moving away. For the longest time, I felt guilty for tearing our family apart like that.

But now that the children are grown and moving on with their own lives, I think it’s best that he and I aren’t stuck in a marriage void of romantic love. We both deserve better than that. At least that’s what I keep telling myself. But there are some lonely moments when I wonder if we could have had a real good life together even without the romance.
I imagine some of those who marry their spouses siblings do develop romantic love for one another. But I think more often than not it's just a matter of convenience and later regretted. Proceed with caution is the only advice I can offer.