Am I morally obligated to remind my Catholic friends and family about a holy day of obligation if I think they’ve forgotten? The other day I went to Mass for the Feast of the Ascension by myself, and I’m pretty sure nobody else in my family went.
If you have reason to believe a Catholic has forgotten that it is a holy day of obligation, it is a kind thing to do to remind him, but you are not required under pain of sin to do so.
As for Ascension Thursday, in most parts of the United States the obligation has been transferred to the nearest Sunday. If that is the case in your area, then your family was not required to go to Mass on Thursday, May 17; they were only required to go the following Sunday, May 20.