Dutch Law (Gelderland) those days (about marriages of people who were related in some degree)
At the " Echtordnung ordinance on the marriages off " 26 May 1597 by governor Chancellor and councils issued by Gelderland , alongside enforcement of before provisions also the marriage between first cousins was not allowed/forbidden.
(but they could ask for an exemption though, so they had to ask for permission for these kind of marriages, so there was a law but it was more a law on paper, because many cousins and otherwise related, people still got married or got married already at another place while this procedure (so if it was alowed) still had to take place)
So this ordinance , was re- enacted in 1627 and 1658 (they kept trying ;) ) , and was on October 9, 1660 replaced by (again) a new one in which not only the marriage between first cousins (Art. X) , but also different degrees of relationship are listed , between which a wedding was forbidden , including the marriage of a man with the widow of his brother or sister of his deceased wife (art. XV) , a man with the widow of his brother descendants, or sisters (Art. XVI) and a man with the daughter of his deceased wife's sister (Art. XVII) So in Holland they kept trying to prevent these marriages , but many marriages still took place and were alowed (, ''money marrying money'' so with cousins for example to keep the money, goods/ houses/castles in the family of course,)
So like Philip says it very well could be they were somehow related , uncle and cousin even, those days more (for us and nowadays) very strange things happened , but people back than didn't seem to bother as much as we do now , because it happened all the time
Also tried to decipher the marriage record ;) , to see if maybe the parents of both were mentioned and where they were born etc.:
I guess (a really wild wild guess for some (read most) of it of course with this handwriting :P ) and with the help of the Paleografy alphabet Jan provided, by the way the M is missing ? ) is that it says/ mentiones (something like) :
The herefore mentioned Roeloff Janssoon of (could be Marstreant (but many other things to me , like Marstreaul or Mentreaul ?) than occupation or ?? (no idea, and can't make anything out of it) age 21 years , (geen ouders hebbend(e)? has no parent(s ?) than a stricked word, so maybe started over here : Assisted by Jan Gerritsz his ? (zijn ?) living at the (woont bij de) St Thonispoort (?) and (en) Anna Jans of Vleckeroy bbe(maybe short for afkomstig van?) Norwegy age 18 years, assisted by (geassisteerd met) Trijn roeloffs her (haar ?) mother (moeder) (woont als voorzegt ? lives as mentioned before? so same adres?)
Well that's all I can make of it , and it doesn't mention they are uncle and niece or that Trijn Roeloffs is his sister, so we need to do some more research I guess , and check what and if something is wrong with the lineage/ connections by comparing a few genealogies, as much info as we can get I guess eeh ;)