Finnish question

[tiia]

Hey,

I encountered forms like "siskosein" and "sydämessäin". It is clear to me that in those cases the meaning is "siskoseni" and "sydämessäni" and that it is not there by mistake. So I'd like to know in what cases you switch the -ni ending into -in and especially what kind of impression does it give to the reader/listener. Where do these forms come from? Do they sound a bit more old fashioned?
Can I also do this with other personal endings, e.g. with -si? (With the other personal endings I somehow cannot imagine how such a form would look like.)

[Serpent]

Yes, that's old-fashioned/poetic/epic. Sometimes it can be seen even in modern song lyrics. People might also use it jokingly.
The equivalent with -si is just to drop the i, but that's very different stylistically nowadays.
I don't know about the origins, seems like simple inversion to me. Maybe it's the normal form in some dialect, the way -sj (palatalized s with no i) is normal in Savo instead of -si.

[tiia]

Thanks you two.

Btw. "sydämessäin" and a few other examples were indeed from some song lyrics, but "siskosein" is from a children's book (translated into Finnish in the 80's).