It depends on what's in her contract, or on local arrangements.
In the likes of Civil Service/Health Boards etc if for example your sister is working in Dublin & the funeral is in Dublin or somewhere reasonably close, she would be entitled to a half day compassionate leave. If it was further afield she would be entitled to a day. It has to be a blood relative - by this I mean that your aunt would have to be a sister of one of your parents, & not a sister-in-law.
In a private company, it might be the case that compassionate leave is only provided for parents, partners, offspring or siblings, so she would really need to check with HR Dept. as to what arrangements are in place.