Substitute Sherry for Wine in Sour dough cookies?

Sherry is wine, so yes, you can use Sherry in your recipe. Sherry is fortified, so its percentage of alcohol is perhaps a third more than most other wines. You don't say what kind of wine you usually use, but you could slightly dilute the Sherry with water to give it a close to equal percentage of alcohol as your usual choice. That would be the safest way to make the substitution.

There are many types of sherry, just like there are many types of wines. Alcohol content is one difference: normal wine can be anywhere from 8% to 15% alcohol, depending on type. Sherry can vary in the 15-22% range. Unless you are using a lot of alcohol in your recipe, it's doubtful that it will have a major impact on yeast growth. It may take a bit longer for your dough to rise with higher alcohol, but if the wine/sherry is only part of the liquid, this effect will be relatively minor.

The other main difference can be sugar content. If you are just using something labeled generic "sherry," it's probably fairly sweet. However, sherry is also a term used to refer to a wide variety of Spanish fortified wines, varying from extremely dry fino sherries (usually 15-17% alcohol, not much more than normal wine) to dry oloroso sherries (usually higher alcohol content, but still dry) to the more "dessert wine" type sherries (like "cream sherry" and Pedro Ximénez or PX) that can be much sweeter than any normal wine. A high alcohol sweet sherry might actually add enough sugar to give a slight boost to the yeast and somewhat counteract any minor slowdown from the alcohol.

If I were substituting sherry for wine, I would choose a style of sherry closest to the wine I normally use, if possible. If you use a dry white, substitute with a fino. If you use a dry rich red, try an oloroso, or perhaps an amontillado for a lighter red. If you use a sweet red wine or rosé, perhaps a sweet or cream sherry could work as a substitution (probably most common for cookies).

In general, if there's only a small amount of wine in the recipe, the substitution probably won't make any noticeable difference. If the wine content is high enough to influence the flavor, I'd be much more concerned about choosing a sherry with a similar flavor than I would be with minor differences caused by alcohol content and yeast; usually it might just require a little more patience for the dough to rise.

(If your recipe has a huge amount of wine in it, you might follow Jolenealaska's advice to dilute the sherry with some other liquid to avoid problems with yeast. But that could also dilute the flavor, so I'd try a small batch first doing a straight substitution.)