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There are two differences between твої and Ваша:
First, number agreement. The possessive pronouns (except його ‘his’, її ‘her’, їх ‘their’) need to have the same number and (in singular) gender as the noun they modify. Ваша/твоя are singular feminine forms, Ваш/твій is singular masculine form, Ваше/твоє is singular neuter form, and Ваші/твої is a plural form. Since сестри is a plural noun, you need to use the plural form (твої сестри or Ваші сестри).
Using a singular possessive pronoun (Ваша) with a plural noun (сестри) is a mistake. Ukrainians would probably understand you, but it’s ungrammatical.
Second, the politeness or number of people addressed. Твої was originally a form used for one person (like English ‘thy’), and Ваші was a form used when addressing several people (like English ‘your’). Just like in English, the plural form became used when addressing one person, too (so, ‘your sisters’ became used instead of ‘thy sisters’, Ваші сестри became used instead of твої сестри).
But in Ukrainian, the change only happened partially. In English, no one would say ‘thy sisters’, this form is basically dead. In Ukrainian, твої сестри is still a alive and well. It’s widely used form, but it’s used for your friends, for people you know well, for which you don’t need to use extra formality. Ваші сестри is more polite and formal, твої сестри is more informal and easy-going.
The possessive твої ‘thy’ / Ваші ‘your’ correspond to the pronouns ти ‘thou’ / Ви ‘you’. If you use ти to address someone, then you’s use твої сестри to address their sisters. If you use Ви to address someone, then you’d use Ваші сестри to address their sisters.
Duolingo generally accepts both polite and informal forms, so both Ваші сестри and твої сестри should be accepted.