My nephew got married, so should I get them individual gifts or one joint gift for Christmas?

why? she's nothing to you. only get your nephew a gift. end.

on the other hand, you can also tell him how stupid he is getting married in the middle of a pandemic.

Individual gifts! The more is always the merrier with CHRISTMAS!

An individual gift.               

She’s a part of your family now and should be included if you’re buying for him a $50 restaurant gift card for the couple is fine. If your budget allows or a $25 gift card for groceries. Or a monogrammed Christmas keepsake for the couple. It doesn’t have to be expensive just thoughtful. 

Whatever you want to do 

A joint gift, maybe a gift bag with some Christmas goodies?

Is this your first married niece/nephew that has married?  
You make the decision. What is the norm in your family? Do you give all your nieces and nephews gifts? Do you want to give individual gifts as they all eventually get married. Set the trend now.  

With my siblings we give $50 per person. When the nieces and nephews got married we all stuck with the $50. So as they married they got $50 total as a couple. 
This is what my siblings and I agreed on. 

We don’t exchange gifts with my husbands siblings or nieces/nephews 

One gift card for dinner out (or ordered in) or a place like Target or Amazon  for a couple is fine to do. If you get a wrapped gift just be sure it is for the house or one they both can use.