I think it depend on how old the kids where when the family was blended.

It's one thing for two kids that have been raised together since they were toddlers to be affectionate, another thing for two horny teenagers to become step siblings at age 13.

I seem to remember reading some study that said children that have been raised together since a very young age are not sexually attracted to each other, even if they're not related by blood.

I think she means BD = BioDad.

In my experience and opinion, Dad's usually back off on the physical expression of affection for their daughters when they begin to enter puberty. In the puberty, high school time-frame I think affectionate families still hug goodbye/hello, sit near each other, dance together at weddings, reunions etc. Cuddling, full body contact, snuggling in the same bed is inappropriate once a child begins the transition to young woman.

It seems as if it's only later when young women are engaged, married, mothers that they may re-engage with their fathers holding hands, tucking an arm through theirs sometimes, that sort of thing.

Yes it matters. Oedipal/Electra Complexes are more than just funny Greek stories.

http://www.steptalk.org/node/212331

Shared a thread that I think could be in line with your question.

Here is another that might help.
http://www.steptalk.org/node/79853

Forgive me if I am way off topic but are you referring to the amount of affection between your Sig other and your stepkids? I often have an issue between my boyfriend and his 6 year old son where he, the son, will constantly sit on his fathers lap whether in public, at home... He must be touching him at all times and does not want him to share his affection or attention with anyone. It becomes bothersome in restaurants when his son throughs a fit to sit on his dads lap. Will he grow out of this?