I have MM and I am a  mom of two young children.  I think it is lovely that you want to find ways to help your mom.  I think this is a hard question to answer, though.  Each of is affected differently and to a different extent.  I think the best thing you can do is ask your mom or your dad how you can help. 
Does you mom get bad headaches?  If so maybe you can offer to get her a sleep mask (if she has one) to help block out the light which can sometimes make the headaches worse and a glass of water in case she is dehydrated.  Does she get really tired?  Maybe you can try to make a meal or do a load of laundry.  You might be able to help out with your younger brother (if he looks up to you and listens to you).  You might be able to teach him about cleaning up his toys when he is done or anything he might be doing that makes more work for your mom.  You might also want to try doing some special things with him (taking him to the park, playing games, etc.) to help keep him busy and give your mom a break.
These are just a few suggestions, but like I said, ask your mom and dad what you can do that would really make a difference for them.  I am sure that they would be thrilled just to know that you were thinking about it and that you want to help.  Let your mom know how much you love her and support her.  I am sure that would mean more to her than anything you could do.

LHT,
You are a wonderful woman. tuknits and Mark both gave the best advise. Help out as much as you can when you can. even the little things like keeping you room clean, or helping with the dishes. Your little borther will come around. If you keep a positive attitude this will help your mom know you are looking after her. if you get discuraged we are here for you!!!
huggs,
Becky

Dear Kieu,
I think your daughter is wonderful!!  I can relate to your situation, I have three terrific daughters of my own! I know when I was first diagnosed they were very brave for me but scared about the reality of the situation.  When I left for Stanford they made a picture album for me to take with me because that was the longest amount of time I was ever away from my kids and they remained brave throughout it all.  Now that I am 4 years post-op I wish that patience was still there, now they get agravated at me when I ask them if they have homework to many times, short term memory loss!!   Seriously, LoveHolyTrinity continue to love your mom and try not to worry to much and your little brother will come around soon enough...if you would like to talk to my girls to see how they handled it you are more than welcome to email us personally, my youngest daughter is 14 so she can probably relate to you and your feelings.  Good luck to you sweetie and just know that we are all here to listen and to help if we can, now go and give your mom a big hug!!
Your Friend,
Kathleen