Hello, welcome to the forum, i had three tinfoil barbs in a 5/2/2 foot tank, they outgrew this, ended up having to find somebody with a tropical pond, who drove down from up country to collect, seriously tinfoil barbs need atleast a 6/2/2 if not bigger, if mine got spooked all hell broke loose,Keeping oscars in this size tank with tinfoils will not look nice in the future when they mature, tinfoils can take a time to mature, where as some can grow quite rapid depending on feeding regimes etc,
I would heed the advice given, as we have been there and got the t- shirt, its up to you wether to listen or not, your boyfriends sister with two in a three foot will also need to rehome, It depends really on wether or not you want to be a fish haver or a fish keeper?
Meaning with all due respect, most avid fish keepers like to give enough space to there fish to ensure they have more than enough space to swim around happy, Bearing in mind tinfoil barbs usually grow to around 16inchs or so, imagine that in a tank.
heres mine a while before i rehomed then