Sri Sangom ji,
I had the same doubt as yours. But, I considered the citing - "a brother's love for the sister enables her to find a suitable mate in life" is a mere example of the genes motive of survival and passing on.
Though you have requested Sri Nara with your example, to clarify your doubts on RD's theory of gene survival, I would like to discuss with you, if not mistaken by you and Sri Nara, as how the same theory can be considered ambiguous, considering the sense of hatred or selfishness of a person (openly or through hidden agenda) towards his/her own siblings, other relatives and outsiders, IMHO.
1) Though the parents are fair minded and make attempts to instill love between the siblings, we can find absolute different motive, hypocritical tendencies and selfishness between the siblings, that gets developed at least at a particular point of time.
2) In many families we find Dayaadi fighting more intense and nasty than with other distant relatives or even outsiders
3) A brother's love for the sister can not necessarily enable her to find a suitable mate in life...This, IMHO is highly speculative/irrelevant. A love towards sisters/brothers can only assure love in return in some way and still can not assure consistency.
There are many instances that can expose the uncertainty and confusions in the motives of the genes for survival and passing on.
The love towards family members or to any one in common, can be considered as the genes motive of survival only in the terms of requirements, to be considerate towards others and be considered by others in return, so that the gene can have easy access to fulfill its desires. The genes have a differential predominant motive to find its way towards accomplishment, in general, in any sort of relationships.
Considerations towards own family member, relatives, outsiders and the tendencies to receive in return some way, is the only focus of the genes to keep surviving and passing on. These considerations and gains in return can be due to (with a) predominant motive of the genes as what to give and receive and the ways and means to accomplish.
This is how we can find multiple personalities in each individual. These multiple personalities are the predominant motives (that distinguishes with the acquired qualities of the genes of the parents) of the genes, that helps an individual to apply unique strategy of accomplishments through one self and others, in a different relationships.
With my limited understanding, I could only derive a conclusion to myself that, genes have their own inborn predominant motives & qualities other than the mixtures (through passing on), in order to attain accomplishments of some sort, through some ways & means.