Will my grandmother receive the reward for Umrah in return for her contribution to the marriage of her two grandchildren, instead of going for Umrah, due to her inability to do so for health reasons?

Umrah is obligatory for a person who is financially and physically able, or financially able but physically unable (in which case deputization is permissible). This is the preponderant opinion among the majority of scholars, as is evident among the Shafi'is and the Hanbalis.

If the grandmother is financially able but physically unable, she must deputize someone to perform Umrah on her behalf, provided that person has already performed Umrah for themselves; because Umrah is obligatory, and an obligation takes precedence over a supererogatory act (charity).

However, if the grandmother has already performed Umrah, then the obligation is lifted from her. And if she had allocated money for a voluntary Umrah and then became unable, she is permitted to assist her grandson with his marriage expenses, and she will have a great reward for that, but it cannot be said that she will have the reward of Umrah in return for this action.