Does a brother deputizing for his sister to cast the pebbles (ramy al-jamarāt) due to extreme crowding invalidate the marriage, knowing that casting the pebbles is an obligation, and does that entail an expiation?

It is not permissible to appoint an agent for the stoning of the jamarāt except for one who is incapacitated. Whoever appoints his brother to stone on his behalf, while he is capable of doing so himself, has committed an offense. He must offer a dam (sacrificial animal) to be slaughtered in Mecca and distributed among the poor of the Haram. If he is unable to do so, he must fast for ten days. His Hajj is valid and not invalidated.

If the pilgrim stoned Jamrat al-Aqaba himself and appointed an agent for the stoning on the Days of Tashreeq, his marriage is valid. However, if he appointed an agent for the stoning of Jamrat al-Aqaba on the Day of Sacrifice (Yawm al-Nahr), his marriage is also valid according to the Maliki school and one opinion among the Shafi'is, where tahallul (partial desacralization) occurs upon the expiry of the stoning time. According to the more authoritative opinion among the Shafi'is, tahallul depends on offering a substitute for the stoning (hady), and therefore, the marriage contract must be renewed. Any conjugal relations that occurred previously are considered wat' shubha (intercourse based on a misconception).

Al-Nawawi said: If one does not stone Jamrat al-Aqaba until the Days of Tashreeq have passed, the stoning is missed, and a dam is due. It becomes as if it were stoned in terms of achieving tahallul thereby, but the more authoritative view is that one's tahallul is contingent on offering a substitute for the stoning. Some Shafi'is hold the view that tahallul occurs upon the entry of the stoning time.

In summary: Your Hajj is valid in all cases. You must offer a dam or fast for ten days if you appointed an agent without a valid excuse. Your marriage is valid in all cases, whether what was omitted was Jamrat al-Aqaba or other stoning, according to one of the two opinions. According to the other opinion, if what was omitted was Jamrat al-Aqaba, you must renew the contract, because tahallul is not complete except with a substitute for the stoning.