If you were not the one who is part of the purchase contract, then it is permissible to do such work. If this work is considered despicable to you, as you are serving food that is amongst the firm prohibitions in the Islamic faith, and this would give a negative impression of you in the eyes of people around you, then it is prohibited from this aspect, since it is prohibited for the believer to cause himself to be humiliated. Otherwise, the actual action of transferring prohibited food from the seller to the buyer is not prohibited and taking payment and tips for it is permissible also.
The marriage contract with the mother after conducting a marriage contract with her daughter is not valid; also, marriage with her sister is invalid as long as she is still in Iddah based on an obligatory precaution. After the Iddah end, he is permitted to marry her sister.