maybe your MIL can put the babies down and play some soft classical music in the background. my dd could sleep thru a marching band, because we wanted it that way, we have a big family.
tell your MIL that classical music has been shown to improve math skills later in life.
good luck, you're doing great. and remember, soon your son will be able to talk and tell grandma what he wants.
and he will repeat everything you say to everyone and at the worst possible time.