Answer:
1. lui (to my sister)
2. leur (to our friends)
3. leur (to shopkeepers)
4. -
5. lui (to your mother)
6. -
Step-by-step explanation:
(Les mots au pluriel ont un s)
à + mot(s) au pluriel = leur
à + mot(s) au singulier = lui
Exemple:
Je donne un bisou à ma mère. (I give a kiss to my mother)
I have only one mother (maybe not, in 2020 we dont know) so it's singulier, so lui :
Je lui donne un bisou (I give a kiss to her)
Have à good day !