You have to know the twin's speed relative to one another because each twin represents one frame of reference. I don't think D makes a difference here, it's only c relative to two or more frames of reference. You have three. One for each twin and that of earth. If you're traveling faster than something relative to you then you experience longer units of time relative to the slower moving object. This means that, while 10 years feels the same as 10 years for someone traveling slower than you, they will have experienced a greater period of time for each unit of time you experience.

Let's make some assumptions: Not accounting for acceleration one sister travels at .5c and the other at .99c relative to the earth for 10 years and then come back. Their flight patterns don't matter so long as they are traveling at a constant velocity. I believe there is an actual equation out there that can estimate more accurately the actual difference in time shift but I'll add some figures just to illustrate what happens, not actual figures though.

So both sisters leave at the same time and don't come back for 10 years. The one traveling at .5c returns 10 years older but for the people on earth, they experienced 20 years in her abscence. The first sister waits for the second sister who returns 20 years after the first sisters arrival. The second sister has still only experienced 10 years but for her sister it's been 30 and for the people of earth, it has been 50. The figures are wrong but actual figures would make this same general pattern.

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I am forever striving for increased perfection of my understanding of this world we live in. My quest through life is knowledge and I am but a child on this ubiquitous journey, as are we all. Knowledge, like the universe, is infinite.