dependent brother and sister u16 in jamaica coming to UK

Hello Everyone,
My wife and I are both British citizens and through naturalisations and we both work full time, she is a teacher and I am a team leader at a residential young people's home. My brother and sister are both under 16 and living in jamaica. I am responsible for them as their dad has passed away and their our mom is somewhere in usa....she doesn't look after them.. they are living with my grandma who is not there grandmother as their grndmother is old and sick and cant afford to look after them. i was told that we could apply for them using the VAF4A form. Has anyone used it for similar reasons before and does anyone know what the chances for them coming to the uk are like and also rough processing time. Any comments regarding this would be highly appreciated. Thanks in advance

See also Guidance relating to family members.