If you have been married for more than 4 years, you don't need a spousal, you can just get ILR directly. But if you applied being married less than 4 years, even 3 years and 11 months, you will have to have the 2 year probationary spousal before you get ILR.
OH! Your wife would become British by registration *if* your MIL qualifies, but just being born in the UK does not implicitly confer citizenship like it does in the US. Smaller technicality, but worth bearing in mind.