This paper user evidence from anonymized system log data on all Estonian e-votes from 2013-2015 to examine for patterns and combinations indicative of family voting.
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Using logs we identify unique e-voting sessions coming from the same IP address and computer with the same operating system that happen in close proximity to each other, specifically with not more than 10 minutes between the end of one and the beginning of another unique voting act.
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The results show that 7-8% of e-votes are cast in such pairs. The age and gender structure of these evoters also shows a set of distinct combinations. The age differences in these pairs are either very small or large. The largest group is formed by same aged pairs of opposite sexes, indicating same aged partners e-voting together. Another prominent pattern are pairs with large age differences of same or opposite sexes, indicating a parent voting together with a voting aged youth.

The new minister of Ministry of Economic Affairs and Communications (MKM) Kadri Simson sees this as a concern for i-voting:

“The Estonian Constitution says that the Election must be general and uniform. When the old man votes in the polling division, it is not allowed that his young cousin comes with him to polling booth and helps him to vote. However, in the Internet voting it is quite possible, since there is no control over who is assisting in the use of ID card.” said Kadri Simson, the chairman of the Center Party fraction in parliament.

Links:
https://digi.lib.ttu.ee/i/?6967
http://www.pealinn.ee/koik-uudised/kadri-simson-eestis-pole-antud-voimalust-e-valimiste-turvasusteemi-n174077
http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0177864