Don’t matter who fill out the application, the problem is that the applicant need demostrate at the embassy strong ties to the country. I am dominican and i do paper works for people apply for tourist visa, the applicant need has a good job and make good money to get a visa in Dominican Republic, sounds that your sisters in law aren’t working because nobody working can take a vacation of tow months in Dominican Republic. Many teachers get visa easy in Dominican Republic because they make a decent money, and teachers only have one month vacation, so where do they work that can take tow months vacation?
if they dont have a good job, they will lose their money. You paying for the flight show to the officer that they dont have money to pay for their own.
dominican embassy check their job, how much they make monthly, how much they have in the bank, if they have property, house, cars etc etc, the thing it is the look that you have strong ties to come back to DR.