Hi - interesting question.
Lets break it down a little.
There are 3 keywords - sin, worship and Sunday.
Firstly regards sin:-
I am not convinced that we should consider it sin and condemn ourselves to being a sinner if we do not follow this doctrine or that Dogma. We are all God's children and I am sure that he would not label us as sinners no matter what we have or haven't done. We are what we are - His children. Occasionally our actions may be considered sinful when they go against society or take us away from the spiritual path towards God. But whether or not something should be considered a sin should IMO depend more on rationality than on whether this or that scripture lists it as a sin.

Next we can look at worship:
In order for our minds to move closer and closer to God they should become more and more similar to Godliness. Since God is an infinite entity, we can infer that our minds should be expanding in order to become more "Godly". So worship should include only those practises that expand the mind and bring it closer to God.

Now what is Dogma? Dogma is a belief or idea that stops the mind from expanding beyond it's periphery - i.e. that stops the mind from moving forwards in its march to attain Godliness. So, there is no scope in worship for any dogmatic practises.

How often should we engage in activities that bring our minds closer to God - clearly thge optimum would be all the time. However, we need to live in the world and support our society also, so we should set aside times when we can concentrate more fully on these practises.

Third keyword:- Sunday
Sunday is an astronomical concept - sun day, as is Moon day (Monday). This raises the question - are there times according to Sun/Moon phases that are better for worship? I have found that the moon has a strong influence on the state of mind in relationship to worship. During full and New Moon (spring tides) I have found that the mind is less able to really concentrate. I have found a good yogic explanation for this in that the water and air factors in the body are attracted to the moons gravitational pull and are drawn upwards in the body, disturbing the mind. Resonates with peaks in crime rates and incidences of disquiet in Mental asylums. But I have found that the yogic remedy of dry-fasting just before the New and Full moon counteracts this effect. It is also during these fasting days that the mind, undisturbed by the stomach, is able to concentrate on subtle god-centred practises.

So I would say that there is some biological/atronomical reasons for wanting to have worship days on certain days. However, thre does need to be some objective adjustment and so practically we can say that a 5 day workweek and 2 day weekend is what humans can comfortably maintain in the short-term present situation. Therefore one of the non-work days should be designated a day of worship. I think it would be dogmatic to say it needs to be Sunday or Saturn day (saturday).

Hopefully, in the future when our society progresses towards more God-centred living, these worship days would fall on the beneficial moon and sun phases to allow us to get the most out of them.

So, in short - it would be dogmatic to say it is a sin to worship on a Sunday.