Print the information off below - hand it to your MIL. It's from UNICEF.
Say something along the lines of 'I know you just want what's best for us, but your comments about breastfeeding have made me wonder if you're unaware of the latest research. Here have a look. I think you'll find it really interesting'"
Oh, and kudos to you for doing such a great thing for your dd. 
News
There has been significant reliable evidence produced over recent years to show that breastfeeding is a major contributor to public health and has an important role to play in reducing health inequalities even in the industrialised countries of the world. Baby Friendly provides a review service of papers, relevant to the health benefits and management of breastfeeding. We aim to review those papers which are pertinent to the industrialised nations in order that they are of particular relevance to our audience. When considering these papers it is important to consider the methodology, sample sizes, response rates and reported findings to determine their relevance. Where systematic reviews or meta-analyses are available, allowing analysis of findings from a number of well-selected studies, these obviously provide a stronger evidence base on which it is possible to base arguments with greater conviction.
It is always important to bear in mind the following when considering the strength of any evidence:
Carrying out randomised controlled trials which are clearly recognised as the “gold standard” is not always possible as it is not ethical to randomly allocate mothers in a way which arbitrarily may decide that half of the group will bottle feed.
Many studies are flawed by staff or mothers deviating from the protocol as this may seem counter-intuitive or too hard to follow in the situation in which they are being cared for or living.
Sample sizes, particularly for older infants, become very small, particularly in countries such as the UK with low breastfeeding continuation rates/exclusivity. It is therefore difficult to accurately infer statistics.
In specialist areas such as caring for preterm infants, it is difficult to recruit mothers at such difficult times and again, small numbers are reflected in a lack of solid evidence about many aspects of care.
Breastfeeding may be documented as having a “small protective effect” against certain illnesses. Whilst it is clearly accurate to say that that the risk of not breastfeeding is greater with some conditions than others, what is a small protective effect in one child is likely to have a much more dramatic effect across a whole population.
Below is a list of differences in health outcome associated with method of infant feeding. The studies have all adjusted for social and economic variables. All were conducted in an industrialised setting.
Artificially-fed babies are at greater risk of:
gastro-intestinal infection
respiratory infections
necrotising enterocolitis and late onset sepsis in preterm babies
urinary tract infections
ear infections
allergic disease (eczema, asthma and wheezing)
Type 1 and type 2 diabetes
Obesity
Childhood leukaemia
SIDS
and breastfed babies may have better:
neurological development
cholesterol levels
blood pressure
Other studies of health and breastfeeding:
cardiovascular disease in later life
childhood cancers
breastfeeding and HIV transmission
breastfeeding and dental health
Women who breastfed are at lower risk of:
breast cancer
ovarian cancer
hip fractures and reduced bone density
Other potential protective effects of breastfeeding (more research needed):
for the infant:
multiple sclerosis
acute appendicitis
tonsillectomy
improved parenting
reduced child neglect/abuse
for the mother:
rheumatoid arthritis
maternal type 2 diabetes
postnatal depression