Definitely, a taxi is the way to go, and with your parents being in their 90s, they may not feel up to staying until the end of the night, so they could arrange to leave earlier. If your parents can afford it, then I see no reason why you should offer to pay for it, just offer to book it for them and tell them how much it will be. If they can't afford to pay themselves, then it's up to your sibling to pay for it, as it's their daughter's wedding, and if they want their daughter's grandparents to attend, it's up to them to pay, and make the arrangements to get them there. The fact that they have a wedding to arrange doesn't mean they can't manage to book the taxi for the elderly grandparents!