Is MIL of an age where she paid the 'married women's stamp' as it was expected that she'd be entitled to FILs pension?
Have a look at: www.gov.uk/reduced-national-insurance-married-women to see if that is what you mean.
Alternatively, if you are talking about his workplace pension(s), that would depend on the terms of the scheme, but it is usual that it includes a spouse's pension. My DM gets a lifetime monthly income from DFs pension, even though he died a few years ago, but that could be because he was in a particular industrial scheme where it was proven that historical exposure to hazardous material contributed to his death, so it includes compensation for that.
After the above, if her income is below a certain level, she may be entitled to pension credit, which is a valuable top up, because even if the basic award is low, it is a gateway benefit that gives entitlement to extra help such as a free TV licence and higher levels of cost of living payouts.
Moneysaving Expert is good for signposting to benefit checks and other financial help for vulnerable people on low incomes.
Of course if MIL is in her late 60s and spent her working life as a high flying professional, with her own final salary pension, none of the above may apply, but it's always worth checking that your income is what it should be and you understand what the terms of receiving the money are, both now and when your circumstances change.