Let's try this again: Ok, can someone lay out the possible options? Is there a way for one brother (MC for an alpha run, the other for a NTR run) to get the sister by themselves? Or is the main goal for them to share her?
First of all the game has a few routes.
First choice: What girl?
==============
1) Go for Mia
2) Go for Kasie (Sister)
You can not go for both
Mia Route is easy - Just go for her options when given a choice. Your points with the sister will be too low to do anything meaningful with her and I think she will start doing more stuff with Kenny.
So now we focus on the sister route:
=======================
You basically have 3 routes:
1) Love route
2) Lust route
3) NTR Route (Mia Route is a bit of an NTR route)
If you do not want any or almost no sharing, go for the love route. Just do not make a deal with Kenny, he will be a bit of a pest in the beginning, but will fade to irrelevant later in the game.
For Lust route, IMO it makes sence to make a deal with Kenny, You (MC) still get's the Lion share of the action (90%), but if Kenny helps you, you can raise her Lust stat much higher and can do MORE naughty things with her then on a solo run. I do not really consider this an NTR route as you are still the main guy doing stuff with her, but with some assistance/wingman.
The NTR route can be on the light side if you still try to be in her good books, but pursue an relationship with Mia. Or you can go hardcode NTR (That can ultimately end in a premature ending), by just being an asshole to her all the time.