Visitor Visa Quick Query - going appointment next week

Hello Senior/Moderator,

My brother, his wife and 2 kids ( aged 13 and 9 ) are planning to visit me for Christmas and new year break. All of them are India passport holders but living in Dubai for more than 20 years.

my brother already has a UK 10 year visitor visa. so its only my sister in law and kids applying for their 10 year visas. ( they have already traveled to UK before )

My queries:

1- My brother ( he is also travelling with them ) is not applying for his visa but sponsoring for all the costs of the trip to UK to his wife and kids as my sister in law is house wife and doesn't work. he is giving all the evidence to justify he has funds to sponsor his wife and kids . IS THAT OK?

2- since my nephew is 13, can he apply for a 10 yr multi entry visitor visa. I read somewhere that the 10 yr visa will automatically get cancelled after he turns 18. IS that true? In this case Can we apply 10 yrs for my sister and 9 year old nephew and apply a 5 year for my 13 year old nephew. Please advise

3- Finally we have completed standard visitor visa application for all of them. Is that correct or do we need too fill child visitor visa for the 2 kids?

Your response will be highly appreciated

Thanks again!!