Hi, K.!
I really hope your mom's surgery helps. I had bacterial meningitis myself in Jan. 2008. Thankfully it was caught fairly quickly although I was in a coma for 3 days and had 2 weeks of in-hospital recovery and another 3 months before I was really myself again physically.
As you probably already know, this is a serious infection and the person going through it is rehabilitated the same as a person who has had a brain injury. (And your poor mom has been through this 4 times!) After a brain injury a person's behavior, physicality, and even personality can be altered. (It can make a person "edgey" and more easily frustrated.)
I don't know whether your mom has had a CT and whether this is the case (I mean, whether she ends up with some permanent changes), but it's something to learn more about from her docs.
It doesn't seem fair of your sister to pressure your mom about anything. It takes time to heal and get your faculties back. I found it was very frustrating having to deal with my own mental slowness (in the beginning), the lack of energy, and having to get your coordination and balance back. (You just want your old life back--PLEASE!)
I would urge your sister to talk to the doctors and/or therapists your mom is working with about what she can expect while your mom recovers. If she really refuses, then she should probably limit the amount of time she spends with your mom until a)your mom is more fully recovered, or b)she has educated herself about recovery.
Here's a link to an article about recovering from meningitis: http://www.meningitis.org/disease-info/after-effects . Please be kind and patient with your mom--she'll benefit from your level head.
I wish you all the best--this can't be easy for you. I hope your sister will get over whatever is keeping her from talking to the docs.
Best of luck!