Farmer's tan lines

So the only part that will be visible is the short bit from the Tshirt sleeve to the cap sleeve ? . I think it will of faded some more by the time of your wedding but try asking your photographer if they can do anything to retouch any that show in the photos, I have had to do it many times.

Maybe speak to an expert on self tan about just tanning the "white bits" a little to blend in.

If you've been wearing t-shirts a lot over the summer, that will be why the lines haven't faded - you can still tan even if you don't burn, so you will have been constantly 'topping up' the colour. If you are in long sleeves now, you should see the colour fade over winter. If you start wearing shorter sleeves next spring, before your wedding, I would try to make sure they are the same length as your wedding dress sleeves, and use a high factor cream on your lower arms to prevent further tanning.
By the time your wedding day arrives, the lines should have faded so that they won't be noticeable - and certainly not if you cover them up with foundation. As for people laughing at you on your wedding day - if they're going to do that, then you're inviting the wrong kind of people to your wedding. One of my friends feel asleep in the sun a couple of days before her wedding - she was wearing a v-neck t-shirt and her wedding dress was strapless. She has a bright red V on her chest in all the photos, but she still looks stunning in them, and at the time, none of us even noticed the sunburn, we were too busy celebrating with her.

Here's something, when I have had my shirt off in the summer and get a tan the same as my face. I have noticed that during the winter my face fades faster than my body. it seems that covering up keeps the tan longer and exposing it to rubbish weak sun - like our winter fades it.