Hey guys, I'd love your opinions on what to do with the situation described below.

I have a small (4-5 people on a Sunday) volunteer OF schola which I direct at a small country parish. Our small group rehearses and sings weekly, and we've gotten to where we're pretty comfortable with chant and with singing together as an ensemble. The regulars are not die-hard musicians, but they do a pretty good job. Each year around this time, an itinerant elderly couple who are relatives of one of the schola members insists on hopping on board and singing, both during Lent and during Holy Week. The interpersonal aspects of the situation are complicated by the fact that I've known them personally for years. They're loving, great, and devout people, but their singing is sub-par and they don't seem to realize this to be the case. The result is that music which would normally take us 15 minutes to get squared away during rehearsal now takes two to three times as long to learn, and even that is no guarantee that it will be sung well on Sunday. Additionally, their inability to blend their intonation and stay on pace throws off the other members of the schola. This is frustrating for me as DM, and for my schola which otherwise has been making fantastic progress. I'm fine with them singing hymns (they can manage hymns), but they realistically cannot sing the propers (even simplified ones) or Gregorian Mass settings. Previously, even after asking them to sit out this or that, they've stayed in their seats and sang along as if they believe themselves to know what they're doing. This morning, I had to postpone using a Mass setting the schola has been learning since January because they insisted on singing it, and I had to cantor the propers solo because they sang them so poorly that it sent the whole schola into a tailspin.

How do I politely get them uninvolved without it coming off as some young guy on a high horse treating them rudely? What ought I consider when navigating this situation? Advice is needed.