A lot of step-parents love their step-children as much as they could any biological child of their own. This is especially true when the relationship is formed during the child's early years.
Unfortunately, under the law, step-children don't have quite the same legal rights as natural children. For that reason, you may want to consider adopting your adult stepchildren.
Adoption is often an expensive undertaking when a child is still a minor. Adult adoption requires a minimal investment in legal services because the court doesn't require lengthy (or intrusive) investigations into the home. Usually only one court appearance is required, which makes adult adoption a minimal time investment.
Biological parents may object to adoption of minors and make it more difficult for a step-parent adoption to go through. An adult adoption allows the will of the child to overcome any objections.
Adult adoption can provide your stepchildren with the ability to help you if you become incapacitated. An adopted child is legally the same as a biological child, which means that an adopted child can be considered your next-of-kin for purposes of medical treatments and other important decisions.
If a step-parent dies without a will, or intestate, the adopted child would still be entitled to legally inherit a portion of the parent's estate, the same as natural children. Otherwise, a stepchild could be frozen out of an estate altogether by other heirs or the law.
Finally, the formalization of the parent-child relationship through the adoption process lends strength and legal standing to the emotional bond that already exists. Adoption should quell any questions about your intentions and the legitimacy of your desires if you choose to leave a significant amount of assets to your stepchild.
Regardless of your exact reasons for pursuing an adult adoption of a stepchild, the benefits of one decidedly go in both directions.