Если существительное в единственном числе, то предложение начинается с It's, если в мн. числе - то с They're
далее меняем
кто? на чей?
I - my я - мой
he - his он - его
she - her она -ее
it - its оно - ее
we - our мы наш
you - your вы ты - ваш твой
they - their они - их
It's our family.
They're your sisters.
It's her grandfather.
They're his cars.
It's their garden.
They're its toys.
It's my computer.
They're our friends.