I have done a lot of research on home school programs. I have both a 1 yr old son and a brother who will be entering 3rd grade this year. The brother has been on home school programs in addition to public school ever since he began struggling in Kindergarten. Now he is up at the head of the class and has confidence to spare.

Most people I talk to who home school do it instead of public school and prefer either http://www.abeka.com/A Beka or http://www.rodstaff.com/Rod and Staff because they want a Christian-based program that emphasizes traditional values. That's fine for them, but personally I feel that these programs emphasize bible stories at the expense of academic material. And yes I have perused both materials for K-2nd grade (ages 5-8 in the US) and is just my personal opinion.

Personally I am a big fan of the http://saxonpublishers.harcourtachieve.com/Saxon math program. These deal with nothing but math in such an incremental way that kids are always thinking "This is Easy" and "I can do this". My brother does about 1.5 years worth of material during the summer and spends the school year enjoying himself and socializing while getting what is (to him) basic math review. Yes I realize that such a repetitive approach is not for everyone but it worked wonders with him and he clearly enjoys it. The Saxon material has been consistently more challenging and comprehensive than the material covered during the school year.

A note on usage:
My brother has since kindergarten spent 2 hours every day during the summer (including weekends and vacations) on school work which includes Saxon math and/or reading a book of his choice. At first he was on http://hooked-on-phonics.com/index.cfmHooked on Phonics for reading review but now he reads for fun and doesn't require any extra help. He has done this ever since Kindergarten so he is used to it. At this point with your sister already being 11 you will likely encounter resistance. Remember you will only get out of ANY program what you are willing to put into it. Discipline (on you and your parents part) will be required. In his case he enjoys it since he knows he will be "better" than the other kids as a result.

Also our success is probably due to his actually being quite bright. He was just being under served by his school. We recognized it and took corrective action early.

Good Luck!