I want to lend money for second mortgages to others (through a broker). He suggested to invest the money through a self-directed RRSP. Is this a sound advice (it will lower my income tax, and contribute to retirement, but any downsides?). Secondly, if this is such a great option, why wouldn't everyone do all their business through self-directed RRSP
The Globe and Mail has an interesting article on what you can do with your RRSPs. Be aware that the article is from early 2011 and rules change.
They describe holding your own mortgage inside your RRSP. That is, if you have $100,000 inside your RRSP already and your remaining mortgage is $100,000, you can use that money to pay off your mortgage, then pay back the money at interest, generating a tax-deferred profit inside your RRSP.
That approach may be viable, though you'd want to talk to your accountant first. I'd be very cautious about loaning money to someone else for a second mortgage using my RRSP, though. Second mortgages are inherently risky, so this is a very speculative investment. Once you make an RRSP contribution, that space is used up (barring a couple of exceptions such as the life-long learning plan). So, let's say you used $100,000 of your RRSP to loan to someone for a second mortgage. Any interest payments should be sheltered inside the RRSP (substantial benefit), but if the person defaults on the second mortgage (which you should expect to be a significant possibility), you've lost your entire $100,000 contribution room (as well as, obviously, the $100,000 that you loaned out).
I can't tell you whether or not it makes sense to invest in risky second-mortgage loans and I can't tell you whether, if you choose to do so, it definitely should be done inside an RRSP. There are substantial risks in the loan and there are both costs and benefits to doing so inside an RRSP. Hopefully, though, I've helped you understand the questions you should be asking yourself.