I suspect that one of my sisters is actually a half sister. I know that both my parents have had affairs over the years. She doesn’t look like me or our other two siblings (we’re all between 25 and 32), but she does look a lot like one of my dad's close friends, whom I’ve always suspected had a fling with my mom. I know it’s probably not a good idea to delve into this but I'm obsessed with finding out if she's really my dad's daughter. Help.

He Said: Unless you’re a member of a royal family and the crown is at risk of being worn by the illegitimate daughter of a commoner, you are right, it’s not a good idea for you to dig into this. While there might be some medical benefit to your sister being certain about her genetics, that’s up to her to care about and research. Though your father and his friend may have had legitimate reasons for confirming their genetic contributions, now that you’re sister’s a grownup, most of the parental contributions have already been made.

To answer your question then, the least painful way to approach this is to find something else to obsess about. Frequently focusing your attention and energy on an issue can be helpful, if the issue involves something about yourself you want to change. Although it’s compelling, it is, without exception, a waste of your life to obsess about someone else’s.

She Said: A few things for you to consider. Does your sister have any idea about your suspicion? Has she displayed any suspicion herself? Has your mom ever hinted at who her lovers were at that time? I ask to gauge whether any of these other interested parties have shown any interest in this issue at all, or desire to share their experiences or feelings. It's true that this is very private, but at the same time this  is your family we're talking about. It's not exactly like you're sticking your nose into some neighbor's private life. It's natural to want to know if your father is your sister's father, especially if you've been told that your whole life.

If your sister, mother, or father for that matter, has indicated any desire to delve into this, or openness to the topic when it comes up casually (for instance, when looking through family pictures or offhandedly mentioning "Jane sure doesn't look like us, does she?"), then you might tentatively feel out the matter with an unaggressive question. Softly letting them know that you sometimes wonder about your sister's paternity is more than enough input from you. If any of them are interested further, they will take it from there. In other words, you have a right to wonder, even to wonder aloud, albeit gently since this is a very sensitive and potentially painful question. But unless it's your paternity at stake, you don't have a right to push the issue, demand the truth, or otherwise put a burden on the rest of your family.