So who's the father...

Scientists have discovered that most Marmoset are fraternal twins and that a
very high percentage are chimeras (They contain cells/DNA from their
siblings). Apparently this can lead to strange genetic consequences. For
example a twin can pass on genes from a _sibling_to his own children. In
effect fathering a child that is not genetically his. Researchers found that
in a third of 15 family lines they analysed individuals had passed on their
sibling's genome to their offspring" so it's not rare either... and
scientists think it might also occur in humans...fortunately under far more
limited conditions.