My grandparents have 2 t5’s under a joint bank account, but only my grandfather’s SIN (Canada) is listed.

[MissKriss65]

Canadian Tax:
Both their names are on the T5 but only my Grandfather’s SIN. Before I realized that, I tried claiming the income (just shy of $17,000 CDN) for one then the other and they would collectively owe less money if it was claimed under my grandmother. 

Will the government penalize them if my grandmother claims it instead of my grandfather?

Will they get in 

[TurboTaxSusan]

If the T5 is for a joint account, then you have to enter it based on how much each spouse contributed to the account. Ex: If both spouses put $500 into the investment account, then the T-slip can be split equally. If spouse A only put in $100, and spouse B put in $900, then spouse A can only claim 10%.