My parents said they won’t help pay for the wedding if my fiancé’s sister attends

My(24) fiance(26m)’s sister(34) was once jailed for stealing some food from a supermarket to feed her children.

When my parents found out about this, they said they ‘don’t want to help pay for an event that a thief will be attending.’ I asked my fiancé to disinvite his sister but he asked if we can have a smaller wedding we can pay with just his parents’ help and our money instead, without needing contribution from my parents.

But that would require many adjustments. Am I wrong for insisting that he disinvite his sister?