What is the religious ruling on a well-to-do husband seeking his wife's help in difficult agricultural work, despite his ability to hire laborers, and does the family bear a sin for this marriage?

There is no sin in marrying your sister to this man, as long as she is content with him as a husband and there is no religious impediment. There is also no harm in her assisting him with agricultural work, as long as it does not involve anything religiously forbidden, such as uncovering herself in front of non-mahram (definition) men. If this occurs, she should be advised and informed of the ruling.