There are many kinship terms in Reisu. Most of them come from four roots: Data for father, mama for mother, gati for child and yoti for sibling.
If there are more than one siblings being discussed, or there is a need to differentiate which sibling we can do this many ways. We can divide it by gender saying yotikipa for a sister or yotigatu for a brother. It can also be divided by age, this is usually done with numbers with 1 being the oldest. So yotiha would be the oldest yoti being discussed.
Gati can be used in the same way: gatikipa for daughter, gatigatu for son, gatiha for oldest child, gatitu for second child, etc. Unlike daughter in English gatikipa can be used to refer to any female child, not just ones in relation to their parent. Gatigatu is the same way.
To refer to someone of one generation away we can use the suffix -dua, similarly to how we use 'grand' in English. So datadua is grand father, mamadua is grand mother, gatidua is grand child.
There are some kinship terms that don't use these roots. Most notably koko, which refers to a spouse. Often times this is used as a term of endearment via adding the -xai suffix. Kokoxai is a term of endearment only used in long term relationships.