Sara will stay at Auntie´s house tonight

I think it should be like this:

Bu gece Sara teyzenin evinde kalacak.

Some notes:

teyze + n + in -> Auntie´s ("in" - suffix indicating possessor, "n" is a buffer letter)

ev + i ... -> house (so "teyzenin evi" means "Auntie´s house" ; "-(s)i" is a 3rd person singular suffix, also used to indicate thing possessed by the possessor described before with "in" suffix; "ev" would mean "house" too, but it wouldn´t indicate that it is possessed by someone)

teyzenin evi + n + de -> "in Auntie´s house" ("de" is a suffix meaning "in"; ´n´ is a buffer letter appended before -e/-de/-den suffixes IF word ends with 3rd person singular/plural suffix; if word ends with a vowel, but not with 3rd person suffix, then you have to use ´y´ ).

kalacak -> "to stay" is "kalmak", you remove "mak", append "acak" (because last vowel of root "kal" is "a") and you have future tense form.

 Sara, bu gece teyzesinin evinde kalacak.