We are all wondering wtf is going on in this girl's head, so I did some research (skimmed the LN through google translate).
This manga seems to be promotional material for the 97+ chapter LN. So far chapters correspond 1:1 but without the R18 parts. If the manga is completed at chapter 9 as mangadex says, nothing will be revealed here.
⠀Worth reading?
Not sure since I only read a few chapters to answer these burning questions, and google translate doesn't do a great job, but the R18 scenes don't seem that good. The absurdity of their lifestyle, of which you haven't seen shit yet, could be entertaining.
⠀Why is she doing this:
She has severe androphobia and is just warming up to the realization that she doesn't feel it towards her brother.
At this point she would be embarrassed to show a reaction, touching or kissing would give her away.
When she gets back to her room her hobby is masturbating to herself in lolita cosplay (double meaning title).
It feels even better with a mouth full of cum, she learns, but she doesn't want any on her outfits.
⠀Are they blood-related:
AFAICT yes, AND HE IMPREGNATES HER.
Everyone is so supportive of their relationship that I'm questioning what universe the author lives in. It seems unlikely that it just hasn't been revealed yet since he had a conversation with his parents where they should have brought that up.