This argument begs the question by assuming that it is morally wrong to not value the lives of people who are functionally abstractions, which is the assumption that I am questioning.

No it doesn't. It expresses a position. One that is a necessary exception to the "well I suppose you don't need to understand or care about physics if you really don't want to" concession I had made in the preceding statements. It does convey the message that you disagree with---that physics doesn't care about your disbelief, your sister in a long long time and a galaxy far away is an artifact of the same physics that your are made of and thereby it rejects "Why should I even care?" as an anomaly rather than a default.

For the purpose of the discussion your actual morality is barely relevant and a "prefer my sister to be alive assuming she actual was real according to the physics we operate on" morality is assumed merely for the sake of convenience. If you really want you can outright hate your counterfactual sister, in which case not murdering her could be the mistake you make. Or, you could conceivably construct a utility function that values configurations of the universe only when they happen to be spatially located sufficiently near to the future-object you call "you". That allows you to "just not care" about such things.

I infer, by the way, that you don't value anything in the universe after the time of your death. Some people (claim that they) have such a value system. For you it is implied because "outside your future light cone" isn't defined if you don't exist. As an illustration consider your brother flying at near light speed in the opposite direction to your sister. You can choose, if you follow your brother in your relativistic rocket you can keep him within your future light cone but lose your sister. If you follow your sister then you lose your brother. If you are dead then your utility function doesn't specify which sibling you followed and sophistmagically allowed to remain 'real'.

I suppose you could propose a hack to your utility function such that when you die the "stuff that matters and is considered real" part of reality could become fixed to "stuff that is within the light cone of you at the time of your death". But then that would imply that your sister and brother are both 'real' but that they aren't 'real' to each other.

Because of vagueness, I'm unsure whether or not this would be accurate, but I also get the feeling that you might be conflating convenient societal conventions with actual individual moralities.

Not remotely. Merely assuming one of the must uncontroversial of societal conventions (and human instinct) out of (perceived) convenience. Again, your morality, ethics and values can be as arbitrary as you like for the purpose of this conversation.