Ignorance fought!
This is a lovely idea!
No, a girft isn’t mandatory, but hey, it’s your brother. 
Maybe something marriage-related would be nice, like Welsh lovespoons or something similar that relates to your/the SIL’s heritage. You can get small ones - they don’t have to be displayed on a wall.
A restaurant voucher is a nice idea if they have regular babysitting and like to eat at restaurants and if it works at the restaurants they like. A voucher for somewhere generic like Amazon would be more likely to be used; they might then buy something that they would otherwise have considered too frivolous but nonetheless enjoy.