I'd just like to clarify that in these situations, it's not the sponsor that's rejected; it's the applicant who is. That's evaluated by the visa office, although Mississauga may send them a note telling them to look into it, because they'll be aware of the sponsor's financial situation.
Here is what the manual OP 2 says:
"11. Procedure: Assessment of adequate arrangements
To determine whether an applicant satisfies A39 officers must assess whether the applicant is
able and willing to support himself/herself. If an applicant does not satisfy this requirement,
officers must determine if adequate arrangements have been made for support. In such a case,
the sponsor's situation, ability and willingness to assist and the duration of the undertaking, should
be taken into consideration, even in cases where the immigration officer did not form an opinion
on the undertaking (i.e., when financial requirements do not apply).
The requirements of A39 may be met if there are adequate arrangements for care and support.
This can include assistance from other than the sponsor (for example, other family members in
Canada)."
www.cic.gc.ca/english/resources/manuals/op/op02-eng.pdf
"A39" here refers to section 39 of the IRPA:
"39. A foreign national is inadmissible for financial reasons if they are or will be unable or unwilling to support themself or any other person who is dependent on them, and have not satisfied an officer that adequate arrangements for care and support, other than those that involve social assistance, have been made."
In order to familarize yourself with the criteria used to judge this, it's helpful to have a look at Chapter 1 of "Sponsorship Appeals" by the Immigration Appeal Division (at p. 8, "Inadmissibility for Financial Reasons")
http://www.irb-cisr.gc.ca/eng/brdcom/references/legjur/Pages/SpoPar.aspx
and, especially, to read appeal decisions on this issue:
http://canlii.org/eliisa/search.do?text=%22adequate+arrangements%22&jurisdiction=ca&t=cisr&language=en&searchTitle=Canada+%28Federal%29+-+Immigration+and+Refugee+Board+of+Canada&searchPage=eliisa%2FcourtSearch.vm&sortOrder=date&requestedPage=1
Generally, the government tends to use section 39 with sponsors who have been on welfare frequently and come off of it just long enough to be eligible to sponsor, but it's conceivable they could use it with someone with a really low income. The employment potential of the applicant (i.e., the foreign spouse) is taken into account, such as ability to speak English/French, education level, and past work history, as well as any income sources such as family.
What you want to do is submit enough evidence to convince the government that they would lose an appeal if they rejected you on these grounds. If you intend to claim financial help from family members, in the event it was needed, you should probably get affidavits from them to include in your application.