Can I contest a will if I believe my father was pressured into signing it?

Yes, a will can be challenged on grounds including undue influence, lack of testamentary capacity (that your father wasn't of sound mind or didn't understand what he was doing when he signed it), fraud, or improper execution (such as not being properly witnessed as required by law).

To succeed on an undue influence claim specifically, you would generally need to show the favoured sibling was in a position of dominance or influence over your father (such as being his primary caregiver during his illness), and that they used this position to override his genuine free judgment — courts scrutinise wills that unusually favour a caregiver with particular care.

Such cases typically require medical records regarding your father's mental state around the time the will was made, witness testimony about the circumstances, and sometimes evidence of the sibling's conduct and access to your father during that period. Given how evidence-intensive and time-sensitive these cases can be, it's advisable to consult a lawyer promptly to assess the merits based on your specific facts.