Why мої since you're saying "This is my uncle (singular) with my aunt."? I guess you could simply say "Це мій дядько і тітка."?
This works differently with «з» (with) and «і» (and).
With «з», you can use either, with a slight change of meaning. «Мій дядько з тіткою» adds prominence to uncle: uncle is more important, and aunt is somehow accompanying him. «Мої дядько з тіткою» means uncle and aunt are equally important.
With «і», you need to use plural adjective: «мої дядько і тітка». If you use singular adjective, it will feel as if aunt is not your aunt, i.e. 'my uncle and (someone's) aunt' or even 'my uncle and a woman' (since тітка can be used to refer to other woman, not just to your aunt).
The difference between «і» and «з» is because «і» homogenouns parts of the sentence, while «з» makes «з тіткою» a modifier to «дядько».
this is my uncle and aunt. Both are singular, therefore the sentence uses "this is" not "these are". these are my uncles and aunts. this is my uncle and aunt.