First of all - Lets be clear.
There is no such thing as a Practicing catholic (actively) gay couple.
That being said, if these people are living a celibate life they are not sinning. However it would be better for them and for the community if they lived apart.
The statement you make above that, “This is in a relative’s diocese & they claim that homosexual behavior is not seen as a grave issue there.”
**The Catechism **says this:
Chastity and homosexuality
2357 Homosexuality refers to relations between men or between women who experience an exclusive or predominant sexual attraction toward persons of the same sex. It has taken a great variety of forms through the centuries and in different cultures. Its psychological genesis remains largely unexplained.
Basing itself on Sacred Scripture, which presents homosexual acts as acts of grave depravity, tradition has always declared that “homosexual acts are intrinsically disordered.” They are contrary to the natural law. They close the sexual act to the gift of life. They do not proceed from a genuine affective and sexual complementarity. **Under no circumstances can they be approved. **
2358 The number of men and women who have deep-seated homosexual tendencies is not negligible. This inclination, which is objectively disordered, constitutes for most of them a trial. They must be accepted with respect, compassion, and sensitivity. Every sign of unjust discrimination in their regard should be avoided. These persons are called to fulfill God’s will in their lives and, if they are Christians, to unite to the sacrifice of the Lord’s Cross the difficulties they may encounter from their condition.
2359
Homosexual persons are called to chastity. By the virtues of self-mastery that teach them inner freedom, at times by the support of disinterested friendship, by prayer and sacramental grace, they can and should gradually and resolutely approach Christian perfection.
By the above we can see that the church sees homosexuality as a grave matter. She also declares taht it cannot be condoned in any way.
So
If they are living in sin, and this is being tolerated/glossed over/condoned by the priest then this priest will have much to answer for at judgement. He is permitting a sinful situation to continue without addressing it effectively.
For those who, “claim that homosexual behavior is not seen as a grave issue there”, you might want to show them what the official church teaching is on the matter. That way they cannot plead ignorance.
NOTE: All of my statements are based on the information provided in the OP. None of us can know what the perople involved, priest, the deacons, parishoners etc., might be doing to help this “couple”.