I’m not a lawyer but I’ve worked in publishing for twenty-odd years so I know a little bit about this and a few questions occur. Is it a memoir or a novel? Is the family easily identifiable? Are the people mentioned in the book dead or alive? Essentially if it’s a novel, you will find it very difficult to prove it’s defamatory and if it’s a memoir and the principle offence is caused to people now dead, the law of defamation doesn’t apply. Memoirs often involve creative versions of a person’s life too - you will often find a legal disclaimer about this if you look at the beginning or end of a memoir.
Fundamentally, unless your aunt’s book was a bestseller it’s unlikely to be optioned for film or tv, and if it is it’s unlikely to be made at all, and if it is made it’s unlikely to be in the form she wrote it - names and details almost always change. If you or your family didn’t sue over the book it will be more difficult to take legal action at this stage and it’s likely to be very, very expensive.
If you’re on good terms with the co-author, raise your concerns and ask for specific changes to disguise you and your family rather than trying to block the whole project. Now is the time to make a deal, if they want your permission to go ahead, but meet them halfway as much as you can. Otherwise you can contact the producer directly if it is optioned and in development, and raise specific concerns. But a word of advice - your anger with your aunt is not a reason in law for a deal not to go through. It could actually feed into publicity around the book and adaptation. Try to approach this calmly and set aside your resentment to think about how this might actually affect you and your loved ones. If your main motivation is not wanting the co-author to benefit from this situation, legally and morally you might have to come to terms with your feelings and let it go.