My DD is 11 months old and still doesn't eat a lot of solids. In a typical day she nurses every 2-3 hours, sometimes more often as she likes to snack nurse.
In the morning, she eats some dry Cheerios about 10 of them or so. Sometimes she will have a bite of what I am having for lunch. At dinner time, she usually has about what would equal a 1/4 of a small jar of baby food of whatever we are eating. Otherwise she has about 1/6 of an avocado if what we are eating is spicy or something she can't eat.
Don't worry what other's say. You know you are giving your baby the best. She will eat when she is ready. Abby hardly ate at all at 7 months. She is happy, never been sick, superactive and has hit all her milestones including walking. She is small, but that is heredity, not what she eats.
Follow your heart and your baby's needs. Forcing solids could make her not want to eat them. She will show you when she is ready! Breastmilk is perfect nutrition for her right now.
Your MIL is just from a different school of thinking. I looked at my baby book and was started on rice cereal at 3 weeks *yes, weeks*, fruits and veggies at 2 months and by 3 months pureed meats. I was formula fed. But, my DD who rarely eats and is breastfed has weighed almost exactly what I did at each well check. My dad always says I should give her more, but I am the mom and I know he is just uninformed. I have tried to educate him, lol.
I almost think that solids are a waste anyway. Half the time I see them in her diaper, undigested. I think it is just a learning experience for them and tastes, but not for nutrition. Especially for a breastfed baby.