Hello! First, it's better "onee-san" (お姉さん) and "onii-san" (お兄さん). You need "o"(お) top of these.It sounds more natural and polite. And I think it's not popular for strangers to refer to each other as お姉さん and お兄さん. And we use these after being closer or after making any relationship. Second, we don't use "otouto-san" (弟さん) and "imouto-san" (妹さん). If the speaker is older than the person being addressed, we use "#%&-kun"(くん) and "@#&- chan"(ちゃん). For example, if you are older than me, you can call me "Hana-chan". We use 弟さん or 妹さん, when I wanna say "How about your younger brother /sister?", "弟(妹)さんはどうですか？" for example. Third, I think this is Japanese characteristic point, we don't use subject if we are not sure how the relationship is.

you might call your (soon to be) wife's father お父さん. other than that, it would be very weird. and even then it would be a bit strange.