When my husband came out of rehab, we were asked to friends for dinner. He said that it would not be a problem if people drank at it. It was too soon - he did not drink but he has not gone out since- 6 months later. Although he has never admitted it, he found it excruciatingly difficult.
I don't think you are being unreasonable- perhaps you could show support by refraining,and hopefully your relative will fools suit.