When you are watching someone make a bad financial decision — and the person is intent on plowing ahead despite your warnings — sometimes all you can do is be a witness.

I’ve dealt with dozens of people and their financial issues, and I’ve learned that a lot of times your only option is to let them fail and, in some cases, suffer the consequences.

In a recent column, I addressed a question from a reader about her 30-year-old sister, who has moved into their parents’ house along with her boyfriend. The couple doesn’t pay rent, despite a request from the parents to do so.

“My mom’s main argument is that she doesn’t want to kick someone out in the cold,” the reader said.

In my advice to her, I said she was right to be concerned. But in the end, she can’t force her parents to see what she sees — a spoiled adult child taking advantage. I told her to just let it go. It’s the parents’ money to do with what they want, even if it’s to enable a grown woman and what the reader described as her trifling, unemployed boyfriend.

[She’s 30 and lives in her parents’ basement. Should they support her boyfriend, too?]

Well, some readers thought I let the parents off the hook. Others didn’t think I sympathized enough with the responsible sibling. I’m always open to hearing alternative viewpoints, which is why I created a feature I call “Talk Back.” It’s a chance for readers to share their opinions.

One person said I was too dismissive in suggesting that the responsible sister just accept that her parents have a right to do what they want with their money.

“I get that the parents’ money is theirs,” another reader wrote. “But what happens if the support for the sister spends down their assets and they need to ask the sibling for financial help in retirement? Unfortunately, far too few people have the savings to support themselves in retirement. Is it reasonable that the sibling then helps her parents without resentment?”

I assure you, I understand the point that the parents, in supporting one adult child more than another, can create tension between the siblings. It can lead to a hostile situation that may endure long after the parents are gone.

Many fights about inheritances stem from siblings who “did all the right things” feeling they were cheated because their parents unfairly supported irresponsible adult children.

[How to keep your cash from leaving the nest when your kids do]

But if you’re an adult child who is not being treated equitably — in your opinion — you do have a choice. You can get angry and, as a result, alienate yourself from your siblings and/or parents. You can decide whether you’ll live a life full of bitterness for money not spent on you.

Or you can take a more emotionally healthy approach that doesn’t negate your hurt feelings but that moves you on to a place of serenity.

Having watched my own mother, who left me to be raised by my grandmother, play favorites, I decided — after some very good therapy — to let the bad feelings go. I chose to forgive her — not for her but for me.

No matter how much my reader points out to her parents that they are being taken advantage of, they still continue to enable her sister and the boyfriend.

If you have no power to control how your parents dole out their money in a situation where they are not physically or mentally ill, such as suffering from dementia, you have no choice but to stand by and let them do what they want. You have no choice but to watch them take care of a financially irresponsible adult child.

People have the free will to make bad financial decisions. (Being duped by an illegal scam is another story.)

Could the spending on the sister jeopardize the parents’ retirement safety net? Absolutely.

However, if the sister, who has warned her parents about enabling her sibling, wants her parents to suffer the consequences of their poor decision, she shouldn’t bail them out.

But if the responsible sister resolves to assist her parents if they need financial support, I would argue she should do so without resentment. This doesn’t mean she can’t be frustrated at having to make this difficult choice.

Yet she should be aware that, in coming to her parents’ rescue, she will be doing just what they did — not leaving a loved one out in the cold. And she will be rewarding their unwise financial decisions. Hopefully she’ll see the irony.

Read more:

Advice for retirees rattled by Trump

Your money, your honey: Baby boomers are more likely to keep financial secrets

The upside to clearing the clutter in your life

Write Singletary at The Washington Post, 1301 K St. NW, Washington, D.C. 20071 or michelle.singletary@washpost.com. To read more, go to http://wapo.st/michelle-singletary.