We can see here that the completed sentences with the most appropriate demonstrative adjectives or pronouns are as follows:
1. Esta casa es de mis abuelos, pero esa es de mis tíos.
2. Mi primo es este muchacho que está aquí.
3. Mi hermana no quiere comprar estas sillas, pero quiere aquellas que están allí.
4. Mis padres trabajan en aquel edificio allá lejos.
A demonstrative adjective is a type of adjective used to indicate and specify the position or location of a noun in relation to the speaker and listener.
Demonstrative adjectives point out or identify a specific noun by indicating whether it is near or far in distance or in terms of time. In English, demonstrative adjectives include "this," "that," "these," and "those."