About how to interact with people in Vietnam. we use the expression "Xưng khiêm hô tôn" (literally call yourself with humble words, call others respectfully). so Mr. that Liên used is an English equivalent to Anh/Ông/Ngài/Bác/Chú/Cậu/.... (Big Brother, Mister, Sir, Uncle). Sorry for Bác/Chú/Cậu only has uncle as English words. We really differentiate the male from families. When a marriage happens, the 2 families become connected (hence we call it Thông gia). Now we come to some cases when our father or mother has brother/sisters in marriageable ages, and a child are expected to remember these things at heart:
1. If they are big brothers/sisters in either mom or dad's families, the spouses (both of them) are henced call as Bác/Bá
2. If mother has a little brother, he marries then the spouse is call Mợ, while the brother is called Cậu (Aunt and Uncle, with the Uncle on mother's side)
3. If mother has a little sister, she marries then the spouse is called Chú, the little sister is called Dì (Still Aunt and Uncle, with Aunt on mother's side)
4. If father has a little brother, he marries then the spouse is called Thím, while the brother is called Chú (still Aunt and Uncle, with Uncle from father's side)
5. If father has a little sister, she marries then the spouse is called Chú, the sister is mentioned as Cô (still Aunt and Uncle, with Aunt from father's side)
What mess up this thing further, is that this rule should be applied when our grandparents and great grandparents have children and grandchildren; so cousins from both mother's and father's side should be mentioned just like the above rules (Just swap little brother/sister with cousins)
In wartime periods (we experienced war till 1975), some families are forever separated, leaving orphans behind. Many families adopted orphan children under their wings and the brothers, sisters cared for each other. Later when the adopted child got his/her own family map, two 2 families still call each others obliged to rules, albeit with more freedom