First off, sorry to hear about both your MIL and your brother's illnesses. Obviously the two instances are different in talking with your DD, as there is no reason to share your brother's illness at this time if you feel it would unnecessarily worry her, whereas your MIL's condition is quite different.
I do think that sudden unexpected deaths are often more difficult. When any of us have a chance to see (even via skype) or hear about someone's health changing, it is easier to accept. (Your example of her watching the pets decline seems to capture that.) Getting kids breakfast, so sorry if this isn't worded smoothly...
In my experience, I'd say the most important things are:
1) openness ( talking about changes, sharing -- to some extent -- about your and DH's feelings)
2) Talking about whatever you believe about the meaning of life, death/life after death, what comforts you
3) Giving her space to share her beliefs about death and whatever comforts her, which may be different from your own beliefs
4) is there anything she needs to say/do? (Even if this is a letter to her grandma, or words at her casket, if there's not time for her to talk with her before she dies.) Simple rituals, sharing pictures and story-telling can be quite powerful.
5) Offering a variety of ways for her to continue to process/express her feelings, whether talking, art, journaling, etc... All of which is helped by you and DH being a bit transparent about how you are processing your own, and what provides comfort to you.
