there have never been any studies done showing that babies sleep better after starting solids. in fact the studies to date have revealed the opposite in some cases.
Solids in the first year are not the primary source of nutrition - there is a good saying that alot of people follow "food is for fun before they are one". There are certainly nutrional advantages to solids as well but the main source of nutrition would either be breastmilk or formula during the first year.
There is no way i would be giving an 8 week old baby anything other than breastmilk or formula. The earliest solids would normally be introduced is between 4-6 months.
Here is some information that you could show your MIL with regard to the old wives tale about solids helping your baby sleep through the night.
A newborn is not designed to sleep through the night at any rate, their metabolism is such that most babies will require frequent feeding…a fact that your MIL probably isn't aware of…i know my MIL wasn't.
Good luck in either convincing her she is wrong or just ignoring her all together.