Just to reiterate scores without %-ile are meaningless...Mensa uses Culture Fair (top score 183) and Cattell B (top score 161) tests in the UK. Your percentile score is expressed according to result and if above 2% you get in. Simple.
Extract from the Mensa website: "Comparison of IQ Tests: As different IQ tests were developed, each was given its own scoring system. Therefore, an IQ of 150 is a meaningless claim unless you know the actual test which was used. In order to compare one IQ test against another, the scores are converted to 'percentiles', i.e. where a person's score falls in comparison to the rest of the population by percentage. Mensa offers membership to anyone whose IQ score places them within the top two per cent of the population, no matter which approved test was used. A top 2% mark in recognised IQ test qualifies you for entry to Mensa. IQ tests accepted by Mensa include the Cattell III B, Culture Fair, Ravens Advanced Matrices, Ravens Standard Matrices, Wechsler Scales and Stanford Binet (this list is not exhaustive)."
mensa.org.uk/what-is-an-iq-test
High-IQ kids (and by that I mean very high IQ of 2%-ile or higher) are, supposedly, frequently (but not always) "let down" by a standard schooling system because they are wired as non-standard by definition as well as what PP have said about frequently co-existing LDs (2e or "twice exceptional" concept). So, OP, it's perfectly possible your MIL is remembering correctly.
I took the 11+ in the mid-80s. It was an IQ test pure and simple (composite of culture fair as NVR/maths and cattel B as VR/english). Food for thought!