My impression of the tradition of fostering is that it could go either way. Sometimes your foster sibling was someone you met when you were a little older child and were encouraged to retain some distance from, so you could grow fond of the person and eventually see them as a partner, not a relative. But in other cases - like, for example, if the parents of one child were busy, at war, or had no interest in raising their child - the child might be basically raised as part of the family from a very young age. So it really depends on stuff that we don't know. In any case, it's pretty clear that the intent was not that it be creepy, and the women got together after several years apart, so in retrospect I'm not bothered.
It sounds like you're equating fosterage with adoption. They were two entirely separate arrangements back in the day. Fosterage was purely political and about creating ties between nations, but it did *not* carry the connotation of becoming part of the family that raised you, even symbolically. At the end of the agreed-upon fostering period, you went home to your birth family. Whereas with adoption, the child *does* become a permanent part of the family unit and takes on the family identity who raised them.
It's confusing, because in modern usage, "fostering" is more like temporary adoption, and sometimes leads to actual adoption, and almost exclusively happens because the child's birth family isn't able to care for them for some reason. At least in the West, we don't really practice fostering for the purpose of strengthening political/social ties anymore.
To use a more mainstream example, look at the Stark family in Game of Thrones/ Song of Ice and Fire. In the Stark household, you've got the Stark children, Jon Snow, and Theon Greyjoy, all of which grew up together. Jon Snow is Lord Stark's son by another woman, so he's basically adopted into the household, and holds a status as both the other kids' blood brother, and as someone not-quite-the-same as the other children. Theon is Lord Stark's foster son, taken from his home at a young age as both a hostage to prevent his people in the Iron Isles from resuming war, and so that he can grow up with positive relationships to the Starks and return home as an adult ready to continue that political tenor. Jon Snow marrying one of his siblings would be squicky, even if he weren't their half-brother. But nobody would blink at Theon marrying one of his foster siblings, because everyone understands his status in the household.