"My friend introduced his sister to me."
Translation:Mój przyjaciel przedstawił mi swoją siostrę.
In english: his(her, my, your, our, their) own sister (feminine), his(her, my, your, ,our, their) own brother (masculine), his(her, my, your,our, their) own window (neuter). In polish: swoJA (feminine) siostra, swóJ (masculine) brat, swoJE (neuter) okno https://en.wiktionary.org/wiki/Appendix:Polish_pronouns
There is a way if Michał introduces Piotr's sister to me. There are far worse sentences in the current version of the course. Most of them are caused by the fact that "swój" has been introduced a lot later than the 'normal' possessives.
We have fixed this in the new Tree 2.0 we're building, but as far as the current version is concerned... it's kinda pointless to find all the wrong ones and fix them. Provided that fixing them is even possible (that the given sentence is introduced after the introduction of "swój").