"Stachowskij" it can be an Ukrainian surname
Rather other way around.... skij is made from Polish -ski.
Yes, you may be right (that's why I added "the other way around") . I didn't look at the surname origin just concentrated at the question.
I will get the invitation and look at it again. The date was like 1928
If they (your relatives) were Polish and used to live in (what's is called now) Ukraine or in any other parts of Russia, their surname was changed into Ukrainian/Russian version which ends with "wskij".