I don't know if you still need this answered, but I'll put this up anyway. I've found a way to get siblings to marry each after playing around for a while. It's a bit of a lengthy and confusing process, but I've tried to make it as concise as possible.
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Choose your person, let's say it's Anna
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Add parents for Anna, and let's call them Babs and Carl
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Add parents for Babs, and they're Dolores and Ed
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Add a new partner for Babs, and his name is Felix
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Add a new partner for Dolores, and he's George
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Change parents on Felix and set his father to George
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Add mother on Felix and select Dolores
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Delete George
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Add father on Felix and select Ed
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Delete Carl
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Add father on Anna and select Felix
I hope this made sense and I tried to make it as straightforward as possible. Again, I don't know if you still need this or not.