Susan Thompson Underdahl is a clinical neuropsychologist in Grand Forks, N.D., who recently wrote a chapter on adult sibling reunions for the book "Siblings in Adoption and Foster Care." She has also written a young-adult novel on the topic.

At age 25, Underdahl searched for and found her birth parents, who had placed her for adoption when they were in high school, and they had since married and had two boys and a girl, teenagers at the time.

"It was probably hardest on my sister," Underdahl said. " She wanted to be a psychologist, too, and here I show up with my Ph.D. I think it messed with her identity."

She said their relationship has been "up and down" in the 17 years since then. Her advice for adoptees hoping to become part of a birth-parent family:

Adjust your approach by whether they knew of your existence beforehand. "If so, then some acceptance has already happened," she said. "If it's a surprise, everyone's shocked at first and there may be a honeymoon period. But everyone is internally sorting out their feelings, and a lot of times they get more protective of their mother, because she is going through all these emotions."

Family emotions and behavior may regress. "A family is like a mobile -- if you add or subtract an element, it upsets the balance. Were you the only girl and now there's another sister? How are things evolving between the adopted child and the birth parent?"

Take your time; don't rush it. "Recognize that this is an evolution. You're not going to bond overnight. Time really does make a big difference. How things seem at the outset aren't anything like what they become over time. If everyone's committed, the relationship will move forward.

Get everyone's feeling in the open. "Easier said than done, but communication is so important. Don't stew, and don't think your own feelings are wrong, even if they're negative. You might get very different styles of communication. In my family, everyone was always very polite, no one got mad. In my birth family, they get mad and don't talk to each other for a couple of years."

Get counseling. "Professional assistance can help you sort out confusing feelings. An objective perspective can help when the feelings get overwhelming."

Katie DeCosse, who was interviewed for the accompanying story, had these tips:

• Be prepared for any outcome; a thick skin is helpful!

• Don't join the fray too soon.

• Keep in mind that the only person who has signed onto a relationship with you is the one who found you or whom you found.

• Do not try to become part of this newfound family; some of your new siblings might not want the family to get any bigger. And you probably have a family that is yours.

• Most adoptees are usually the oldest child of the shared birth mother. This can be a threat to the one who has always considered him/herself to be the oldest, so be aware of this.

• Patience, patience, patience.

• Be open and receptive to whatever these new siblings are willing or able to offer in the way of a relationship.

KRISTIN TILLOTSON