I was on vacation with my family and late at night I really wanted a can of soda that my 10 year old sister had. My husband offered her two cans the following morning if she gave me her can at night (we were not convenient to any stores). My father mentioned that it might be ribbis. What is the halacha?
As your father rightly noted, the prohibition of ribbis applies to items as well as to money, and “borrowing” one can with the promise of giving two in exchange is a case of ribbis. Among neighbors or friends, it is permitted to borrow an item and give back a similar item, and there is no need to ensure that the items are identical, because among neighbors it is accepted that small differences are foregone. However, the case of one can for two is a clear cut case of ribbis, and is therefore forbidden.
In this case, however, the age of the sister (beneath bas mitzvah) must be taken into account. Because she is a minor, the validity of her business dealings depends on the rabbinic enactment whereby the transactions of older children (from the age of pe’utos–between six and nine) are valid. Because the transaction of ribbis is prohibited, a minor’s transaction is not valid, and therefore the full prohibition of ribbis does not apply (see Shulchan Aruch 235:1, and Nesivos Ha-Mishpat). Furthermore, the minor does not have a prohibition of lending with ribbis, and there is no full prohibition of paying ribis.
However, as the Gemara (Bava Metzia 73) and the Shulchan Aruch (Yoreh De’ah 160:8) rule that one may not make interest-related transactions even with one’s own children.
In addition, note that a prohibition is only possible where the can actually belonged to the sister. In a general family environment, children don’t actually ‘own’ the food they have, and a mother can (under most circumstances) persuade her daughter to “give back” a can of soda by promising two cans tomorrow, because the can doesn’t really belong to the daughter. This depends on the precise circumstances of the case.