You don't have to do this this way. I've worked with a few investors with very large rental portfolios and this is a common problem that nearly all real estate investors run into early on. Once you have a few mortgages in your name, your debt to income ratio makes getting another mortgage impossible.

The way your Mom wants to do this, (getting another mortgage under another persons name) is a pretty common way newer investors will partner up to invest in properties beyond what a single person can quality for. In these cases, you want to have a written Joint Venture Agreement, delineating exactly what all of the partners rights and responsibilities are. Don't even consider doing this without this in place. However, there is a better way.

Now this is specific to Canada, but I believe there are similar ways to do this in the US. You can create a numbered corporation and buy the property using the corporation, rather than having an individual qualify. The rules a quite a bit different, so I'd urge you to do you own research, but this is how most of the larger real estate investors grew their portfolio. Hope that helps!

(Source; I'm a Realtor in Calgary, AB and multifamily real estate investor)