It's obvious why Mary wants the policemen to eat the leg of lamb, but why is she so keen for them to have some of the whiskey?
There are several possibilities. First, she could simply be trying to endear herself to the detectives. After all, the nicer she is to them, the more they may stop considering her as a suspect. Second, since it is against police policy to drink on the job - especially a murder scene - maybe she is protecting herself in the off chance that they might find the evidence and accuse her. This would give her evidence to have the case thrown out. Finally, the more relaxed the officers become, the more likely they will be to stay for dinner. After they have had a drink, they will be more likely to be open to staying for dinner and helping Mary get rid of the murder weapon.