The stereotypical stepparent hatred and drama displayed in movies is not typically the case. Stepparents often make a significant effort to welcome their stepchildren into their preexisting family and also become organically integrated into theirs. Stepchildren can create very strong bonds with their stepparents; so much so that they wonder how to include them on their wedding day along with their biological parents.
If it feels right to you, consider asking your biological and stepfather to walk you down the aisle together. This is very symbolic so ensure it is something that you truly want.
Ask you blended family to wear colors that coordinate with your wedding palette. This can extend to both stepparents and siblings. This connects them to the wedding and separates them from the other guests. You can also invite your stepsiblings to actually be a member of your wedding party (bridesmaid/groomsman). Be sure to also get photography shots of your blended family as well if you'd like.
Consider doing multiple parental dances with both your biological and stepparent. This acknowledges them both individually on your special day.
Doing a first look with a family member is becoming a very popular trend. If this is something you would want to do, consider involving both your biological and stepparent.
If you are becoming the stepparent...
If you are joining a family as a stepparent, there are a few gestures you can make to go above and beyond to include them in your special day.
Consider inviting them to be a part of your wedding party (if you plan on having bridesmaids/groomsmen) so that they can help during the planning process and also stand with you both at the altar.
If you are not planning to have a formal wedding party, then simply welcome them to wear colors that coordinate with the wedding palette/style so that they feel set apart from the other guests.
Reserve seats in the front row for the children and immediate family on both sides.
Consider giving gifts to the children to symbolize a promise/commitment to them.
Consider giving separate vows to the children to verbalize this promise.
Consider incorporating the children in your unity ceremony. For example, if you like the idea of a sand pouring, you could pour different sand colors to represent not only yourselves, but also each child.
Plan special dances with the children.
Something that could be done no matter your role of the blended family, is include them in activities throughout the day to make sure that the guests feel how special they are to you. You can also invite them to join the wedding parties while getting ready, whether that be hair and makeup or just getting dressed together. Even invite them to wear something similar to the rest of the bridal party. Along with the outfits, you could have corsages and boutonnieres for them as well. Just like the wedding party, there can be an assigned table near the sweetheart table that the families can sit at. You can also invite them to have a special dance on the floor together and allow for each parent to dance with the couple, or even the couple dancing with the children to one of their favorite songs. Creating specialty drinks for your blended family is also a fun idea. You can create a special cocktail (or mocktail for children); one for each parent and/or child.
My biggest overall suggestion is to simply treat them as normal members of your family. Think about what would make them and you happy on your wedding day and/or the gestures you feel your stepparent(s) would really appreciate. The wedding is not only bringing together the couple, but your families as well so take special care in giving representation.