Q. My sister and I are increasingly concerned about the behavior of our 62-year-old brother and his 13-year-old daughter, the only child from his second marriage. She is academically successful but emotionally stunted.

All her friends, as well as her family, belong to a religion whose rules are pretty strict. When she starts dating, she must go out with someone who belongs to her church along with a chaperone, and she must remain a virgin until she marries. She already acts in such a sexual way, however, that I'm not sure that she can.

We physically pulled away from our father and our brother in early puberty, but this fully developed, buxom young niece sits and squirms on her father's lap and lavishes him with kisses and cuddles as though he were her boyfriend. He never asks her to move. When she's not on his lap, she hangs her bare legs over his and he lightly tickles her all over. Or she drapes her arm over his shoulder, strokes his ear and pokes her fingers inside.

We have also watched her stroke, kiss and groom a girlfriend in a demonstrably sexual way, perhaps because she has always liked to cuddle with her parents.

We haven't told the parents that these displays of affection seem provocative and sexual to us, because there is no history of sexual abuse in our family or any temptation to cross that line. We haven't even said anything about the short shorts and the makeup this child wears, since her mother is defensive about any parenting criticism she gets.

In any case, they probably wouldn't believe us, because we don't share their religion and therefore are outside of their trust circle. My brother might even banish us, the way he banished his daughter from an earlier marriage, as well as his two stepsons, after they left the church. My sister-in-law keeps in touch with her sons, but he hasn't talked with this daughter in 10 years.

We don't want to be banished, but we must protect our young niece. Or are we overreacting?

A. No, you're not overreacting.

Every little girl flirts with her daddy from the day she is born, but nature usually makes him pull away from her when she reaches her preteens and certainly by 13 when her hormones are sending out sex signals as fast as possible, and she doesn't even know it.

Your niece probably wouldn't act so seductively around her father if he had ever abused her sexually, but some boundaries must be set - and soon. The situation could escalate at any time.

Although someone, somewhere, may have taught your niece to flirt in such a grown-up way, no one probably told her that short shorts might attract the wrong kind of attention at the mall or that kisses and cuddles might tempt her father to give her more than kisses and cuddles one day.

Because your brother hasn't set any boundaries and since he doesn't trust anyone outside his church, you and your sister should deal with your nieces instead - both of them.

Start by asking your brother whether his 13-year-old can visit you or your sister once a month. He'll probably permit this if you promise to take her to her church. You might invite another 13-year-old to visit while she's there, so she can see that there is a better way to dress and act. Also set some time aside to knit, garden or sew with her, or simply talk with her in the dark after she's gone to bed. Children will confide in you if you leave long silences for them to fill and if they don't have to look at you.

You or your sister should get to know the older niece, too, and when she's comfortable with you, tell her baldly and honestly about your concerns. Ask her whether she thinks her dad might sexually abuse her little sister and whether he ever abused her. If he did, find out whether she would be willing to talk with her sister so she will know how to act around their father to prevent things from getting worse.

If there was no abuse, however, you'll have to tell the 13-year-old - in a kind and straightforward way - that it's embarrassing to see how she acts with her daddy, that it looks as though she's trying to arouse him, and that her behavior isn't okay. And neither is his. Fathers aren't supposed to tickle their teenage daughters or accept sexual attention from them.

Sometimes someone who lives in a sheltered environment has to be told what is acceptable to the rest of the world and what is not.

Questions? Send them to advice@margueritekelly.com.