I [32F] need advice on how to communicate a recurring concern involving my fiancé [33M] and his sister-in-law.

My question:
What is the best way to communicate an ongoing relationship concern to my fiancé in a calm, productive way?

My fiancé (33M) and I (32F) have been together for 2 years and engaged for 1 year. We are long-distance, and his sister-in-law has been in his life for over a decade.

Over the past couple of years, I’ve noticed certain interactions between them that make me uncomfortable. Any single moment wouldn’t matter on its own, but together they’ve created a concern I feel I should bring up.

For example, he sometimes does small gestures for her that he’s never done for me, even when I’m present. At gatherings, he tends to focus his attention more on her than on anyone else. In conversation with me, he mentions her frequently—even in unrelated contexts. When taking photos or videos at family events, he often starts with her and lingers before capturing anyone else. Because we’re long-distance, I don’t see their regular interactions, but he visits their home several times a week.

I don’t want to assume anything about his intentions or feelings. I only know that these patterns have left me feeling uneasy, and I would like to address it constructively.

What I need advice on:
How can I bring up this discomfort in a way that is respectful, clear, and focuses on improving communication rather than blaming or accusing?

TL;DR:
Noticing recurring behaviors between my fiancé and his sister-in-law that make me uncomfortable. I want advice specifically on how to talk to him about my feelings in a calm, constructive way.