As far as sinning, thats the only way she can move on and do EVERYTHING she wants to do. You know how that will hinder her unless she has legal and scriptural grounds for divorce
Nothing obligates you to do something you don't want to do and believe is wrong, just to accommodate choices she wants to make. Her desire to be the wife of a "spiritually strong brother" does not obligate you to go "sinning" just to appease the Watchtower pharisees. Don't burden yourself with all the blame; she is reponsible for her share, and must accept the consequences that come with her own choices.
Further comment and opinion...please decide for yourself if you want to use this approach! The Watchtower uses Matt 5:31-32 and Matt 19:9 to justify their rules. If she or the elders pressure you about "scriptural grounds", cite Mark 10:5-12 and Luke 16:18. These Gospels do not allow the Watchtower's interpretation. Matthew's Gospel uses the Greek word "porneia"--which sometimes means "fornication" or "adultery", but also can be other forms of immorality (such as "incest"). Some translators word this so that Jesus does not advocate that one wrong (adultery) justifies another (divorce), by translating "porneia" as "illicit marriage" or "the marriage is unlawful" in this context. In other words, divorce is allowed only when the marriage itself constitutes an immoral act, so that divorce would rectify the sin--not compound it.