In OP's defence, it is extremely unusual and strange for someone to know three languages but none of it being English. Without giving away identifying details, my work involves a lot of languages and identifying the degrees of fluency. One thing that many, many people tend to lie about is how many languages they can speak.
When someone claims to speak "3 languages", it's usually just one language and they only have very basic or no skills in the other two. Language bragging is particularly common in people with lower education levels but who still want to impress others. They tell everyone they speak several languages because they know you cannot verify if that's true.
True trilingualism in the sense of being able to fluently speak, understand read, write and think in three languages is vanishingly rare. It requires a high level of academics and (usually) some sort of international background. As a result, one of those languages is invariably English as that's the most common denominator across the world. Before anyone gets triggered by the paragraph below, YES there are loads of people in the world who can speak 3 languages truly fluently but over 90% of them have English as one of the three.
So in this case, it's probably YABU not to meet a potential close relative due to communication problems but YANBU to question why she told your brother she "speaks" 3 languages but is in no hurry to learn English. That is a genuine red flag. Based on experience, many people who claim they speak 3 or more languages are simply lying about it impress others or to embellish their CV. It would make you question what other things the are faking about themselves to make themselves appear better.
Lots of people are berating OP here because they immediately believe the fact that this SIL can speak 3 languages. I would say you need to question that first. OP, if your family has never met this girl and you only have what your BIL told you about her, you need to dig a bit deeper to see if she really can do all the things she claims. We don't know more about her family or work background, so maybe there are unique factors that make sense. However simply going from a statistical perspective, it does not sound very plausible.
Might have missed some posts but it sounds like she grew up in Western Europe? English is always the second language taught in schools across Europe so something in her story doesn't add up.