If a cohabiting couple with one child are tenants in common with one owning 75% and the other 25%, what happens when the person with the 75% shares dies? A will made ten year ago has left everything to the deceased siblings.

It depends on the jurisdiction, but in general the 75% individed interest in the property would either go to the heirs of the siblings (if the will contains an anti-lapse provision) or to the decedent's heirs (by the laws of intestacy in the jurisdiction). Usually, I think, the intestacy statute would essentially provide that the estate goes to the decedent's child.

What state is this? When was the deed written?
Generally tenants in common means no survivorship by the other owner, and the will prevails, or the laws of the state if no will.
Joint tenants with right of survivorship is the term generally used when the other person on the deed inherits.