You can contest a will if it was made under duress or undue influence. This needs to have been serious — coercion or intimidation, threats, actual violence, or some form of serious manipulation or deception. The court will ask whether your father probably thought “I don’t want to, but I have no choice”.
Emotional manipulation by your sister, such as appeals to pity or sentimentality or even bullying in the form of persistent nagging, is not usually enough — unless your father was particularly vulnerable to pressure at the time. Similarly, the fact that his will was made very shortly before his death is not suspicious in itself.
Your daughter should take legal advice. A free initial enquiry will help her understand whether she can contest the will and what the best way forward is.