You have to be practical. One must dress differently in church vs at home. Modesty standards apply to both places but it's acceptable to wear jeans and a tshirt at home, while such clothes in church, absent an emergency, would be offensive.
Most clothing sold today is not designed for sleeves to reach the elbow. That's not the style. If one wears such clothing is it a sin? It depends.
In church, yes. Outside church? No.
If one has a short-sleeved shirt to wear to church, then you must put on a sweater or something else to cover more of your body. Most catholics know, and do this.
I don't think that any outside-of-church clothing is immodest if the sleeves are mid-arm. Clothes just aren't made this way.
The bigger problem, by far, is tight clothes and showing off legs. If catholics today were to combat these areas, God would be pleased. I see no sense in arguing about upper arms (except for inside church) when most people are wearing tight/too short clothes.
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