Theoretically, what your MIL is saying is not incorrect. However, sleep experts today indicate that 11-12 hour stretches at night for LOs is more restorative. Her kids likely took longer naps during the day to counter balance the slightly less sleep at night. It's not bad. But not uber ideal for the best restorative sleep. Especially if any of the naps get curtailed in any way.
It was probably the best way for her to manage as many kids as she did to get them on the same sleep pattern.
As pp said, it may be a bit long for A times, but it really depends on your LO.
If you'd like, can you post your entire routine, and maybe we can find areas to help you tweak?
HTH