did she FFed her DCs? Is this the first grandchild as well? One thing you need to be aware of is that often with the birth of the first grandchild, a lot of emotions around their own pregnancies and early days of motherhood are stirred in woman. If she feels slightly guilty that she didn't give her DS and other DCs the best start in life (remembering that a lot of the evidence about the benefits of breastfeeding didn't become widely known until relatively recently, I konw my mum's generation where told there was no difference, and I'm only early 30s), then quite normaly, she might be abit defensive and attack is often the best form of defense.
You might be talking about the health benefits to breast feeding, what you are saying is "this is why I'm doing this for my child" what she's hearing is "you made the wrong choice for your child".
People like this, unfortunately, can be very distructive to your intention to breast feed - often subconciously deliberately ("see, she couldn't do it, it is hard. I wouldn't have been able to anyway even if I did decide to breast feed, I didn't make the wrong choice.").
Make sure you have your DH on side, talk about the benefits, say you're aware it'll be hard and painful but you dont want him or anyone else offering formula unless you decide to do that. You want him and everyone else - including his mum - to be supportive so that you can get BFing establish, if not, you are going to limit how much they are around.
Don't bother buying in nipple shields or trying to toughen up your nipples (slightly concerned how you plan do to that?!), laniosh cream is worth buying in advance (use tonnes! it also makes excellent lip balm!).
oh and I managed a few dinners when DC1 was still bfed, I fed him at 6:30pm, went out, was home for 10pm to feed him. With DC2, even though she's bottle fed now at 8 months, I don't go out or leave her regularly with anyone, I'm not that sort of mother. My MIL hoped I would be (and also kitted out the nursery), but it's not who I am. (Plus we're skint now my SMP has run out, who are these mothers of babies with all the energy and spare money for going out all the time???)