Standard visitor visa for step children

Hello can someone give me some advice about applying for a visa for my wife's children. Her son is 17 years old and her daughter is 20. They both currently live with their family in Colombia and my wife has been here on a spouse visa since 2013. We would like to apply for them to come here to the uk in January 2017 for a 2 week holiday. Can someone tell me what visa we will need but equally important what are the chances of us getting a visa for them. I hear that standard visitor visas from poor countries are almost impossible to get. What sort of documents do we need. Do we have to prove we have the funds now to pay for their flights and all costs here. What sort of documents do we need from Colombia. Her son is still at school and her daughter who plans to travel with him is in university but will have a holiday in January for 2 months. Any advice of any sort would be really appreciated by us both as the rules seem so complicated. Thank you so much

The first thing is to search through the 1000's of posts on here regarding standard visit visas, do some research it will help you in the long run.

It's crucial that you submit evidence to strong ties in their home country.