Maybe they are friends and discussed their thesis topics with each other. I find it unlikely that 4 out of 20 students would come up with sibling related topics independently.
it's generally better to donate $X than it is to volunteer $X worth of your time
In what sense is this better?
Consider a diehard Democrat volunteering for the Obama campaign. He's perfectly willing to spend six hours knocking on doors (for free), because he enjoys spreading awareness and bonding with his fellow Democrats. But to hire someone to do it (for minimum wage), he'd have to donate ~$50, which he might not be willing to part with. So in this case it's much better for the individual to volunteer than donate. It's also better for the Obama campaign, because they'll get more mileage out of someone who did the work for fun than someone who was primarily motivated by the cash.
The only person who is worse off is the hypothetical paid person who Obama would have hired instead. But the volunteer only cares about himself and Obama, not some random person who needs a job.
Remember several years ago, when people were concerned about how unreliable Wikipedia was, and told students never to cite it in their essays? It took a long time for people to trust Wikipedia, and nowadays people only trust it because lots of other people trust Wikipedia. But in the case of Cognito Mentoring, the average customer won't know anyone else who's used the service, nor is it popular or established. It's not impossible for a free service to be trustworthy, but it's a lot more difficult to trust a free service than a paid one.
At most you can do some anchoring. You can tell people how much they should value your product and people can (and often do) disagree with that.
That is true.
Yeah, but nobody's going to think that if the service is offered for free. Some people might if they charge for it. By charging a certain amount you tell people how much they should value your product.
CS grad student here. Some mistakes I made were
Most of the students who we advised said that knowing what they know now, they would have sought advising from us only if it were free.
I think this is precisely because you aren't charging for advice. If you charged a $200/hour "consulting fee," people would see you as professionals, and value your advice highly. But now that you're giving advice for free, you're just some random Internet guy telling kids how to run their lives.
High school and college students generally don't have much money.
Well, you market it to the parents. People pay for college admissions consulting all the time.
Yeah, that sounds like the most likely possibility actually.