A grieving person can appreciate an invitation and still be unable to attend.

Grief makes ordinary occasions unpredictable. Someone can be genuinely glad to be asked and still find, when the day arrives, that they cannot face a room, or the drive, or the moment when somebody says they are sorry. That is not a rejection of you, and it does not mean the invitation was a mistake. Being included matters even when attending is out of reach.

Keep inviting without needing an answer in advance. Say the door is open and there will be no fuss either way. If they cancel at the last minute, take it lightly and ask again the next time.