A couple of my relatives are going to UK for vacation, but they aren't fluent in English. They can understand English but can't speak it. They are going with a group in which people are fluent in English. Will my relatives face any issues at the time of getting a visa from the embassy? And also, at the time of entry in UK, where they will be asked questions like why are you here and all? Also, can they take help from someone from their group to answer on their behalf or will they be sent back if they can't speak English?
This is no problem at all. Yes they can take help from their group members for interpretation. Even if they are alone and require help, the border officials will have interpreters available for commonly spoken languages. Hindi is definitely one of them.
The U.K. does not require standard visitors to be fluent in English.
will they be sent back if they can't speak in English?
Will my relatives face any issues at the time of getting visa from Embassy?
Usually there are no more visa interviews for UK for standard visitor visas. The staff on the application center will most probably be local from the country where an applicant files the application. So they can converse in their native language during application submission.