Answer:
A possible way to fill in the blanks is by using a reflexive pronoun:
Mum and Dad are away this weekend, so we have to look after ourselves.
Step-by-step explanation:
Reflexive pronouns are used when the performer and the receiver of the action indicated by the verb are the same person or people. For example, with a knife in my hand, I can accidentally cut someone. However, if the person I cut is me, I must use a reflexive pronoun to indicate that: myself. For example: I cut myself with the knife.
The reflexive pronouns are: myself, yourself, himself, herself, itself, ourselves, yourselves, themselves.
In the sentence we are analyzing here, the speaker is a son or a daughter. His/her parents are away, so he has to take care of him/herself. However, the sentence uses a plural pronoun "we". So, the reflexive pronoun must be befitting (we - ourselves). Thus,
Mum and Dad are away this weekend, so we have to look after ourselves.