Edit: None of the below is Italy-specific. It would be my general understanding from working in several places across Europe, often along side Italian colleagues who previously worked at Italian Universities. If I am correct in assuming that asking you to confirm whether your co-author is your sibling would be a privacy breach, this would apply across EU. Any Italy specific insight would be great.
As long as none of the publications look out of place on either of the applicants' profiles, I wouldn't expect it to affect either applicant at all. Just make doubly sure you highlight your own name in each of the author's lists to make it easier for the committee to know who is who. And ensure that if asked to describe your contributions, you can do it for each of the publications.
In fact, it might be some sort of a privacy breach to even ask the applicants whether their co-author is their sibling (people with same last names do exist outside of family relations after all!).
If some of the publications look really out of place on one, or both, profiles (i.e. they are on topics which bear no relation to how that person presents themselves), and they frequently happen with the same co-author, this would look bad regardless of whether the applicants/authors share a last name or not. (As it might indicate somebody was "trading" publications and inflating their publication count artificially and not through work and contributions).
However note that if you are applying to a doctoral programme, a fairly diverse publication profile is not unusual (as is no publication profile up until this point), since you are after all still often "finding yourself" during your MSc.