Learn Italian: Entro stasera, le tue figlie saranno arrivate alla meta? - By tonight, will your daughters have arrived at the destination?

Because the main verb arrivare uses essere as its helper verb. Whenever essere is the helper, the past participle has to match the gender and number of the subject. Since le tue figlie is feminine plural, the participle changes to arrivate.

Why do we use saranno here instead of avranno for will have?

In Italian compound tenses like the Future Perfect, you use the exact same helper verb that you would use in the standard past tense (Passato Prossimo). Since arrivare is a verb of motion that takes essere, you form the Future Perfect using the future tense of essere (saranno) plus the past participle.

Why does the Italian sentence use entro to say by tonight?

The preposition entro is used to set a time limit or deadline. While the English word by has many translations in Italian depending on the context, entro is the exact word you need when saying by a certain time or no later than.

I remember that family members do not get definite articles with possessives. Why is it le tue figlie and not just tue figlie?