Risking her life

I'm assuming she is one of your closest friends since she is your bridesmaid. I can't imagine telling my close friend/bridesmaid that she isn't invited to my wedding anymore because she doesn't want to risk catching a deadly virus by attending my reception, but maybe that's just me.

In my area, I can have 50 people at a ceremony but only 10 at a reception. This is because it's easier to enforce social distancing at a ceremony (placing chairs far apart, sitting with members of your own household, no one is touching). The reception gets difficult as people are mingling, sitting at tables together, dancing, drinking, wanting to hug the bride/groom in congratulations.
I think your bridesmaid is making a safe choice, and you should respect it.

I can see both sides of this. As someone who has worked throughout this Covid, I can see where she is coming from

1. She's a cosmologist - She was probably out of work during most of Covid, she's probably back at work and they have procedures in place to hopefully keep themselves and other safe

2. She's your friend / bridesmaid and feels obligated to go and is struggling and not wanting to let you down

At the ceremony she is more likely to be able to safely distance herself from others. At a reception that probably won'y happen (My friend attended a funeral this past weekend and she cares for several "at risk" family members. She wanted to be at the funeral because she felt she needed to and tried to SD herself from everyone without contact, but ONE person DID touch her and now she feels obligated to quarantine herself from her at risk family members). So where I'm going with this is, she can stay in control at the ceremony, but not likely at the reception and at the end of the day, she still has to work and can't risk her health and maybe she doesn't want to risk others either.

You can't be mad at someone for their concerns over Covid and it's definitely not worth it to lose a friend over it. Just try to see it from her point and respect her decision. Afterall, she's still wanting to be there for you at the most important part of your day.... The Ceremony.

If you have a limited number of guests, I can see why you're frustrated. At this point I do think it's better if she doesn't attend. If you're close with her, have a conversation with her and see if you can work something out. If not, I would call/text her and say you understand her concerns and due to that, you think it's better if you and your FI celebrate with her at a later time. Some kind of diplomatic BS.

She's completely allowed to decide she doesn't want to come but she doesn't get to demand an invitation, especially when she's only going to use half of it. She should recognize you have a limited number of people and want to use those invites in the best way possible.

Edit: I want to clarify I do understand where she's coming from, but she should understand this event isn't about her. She should be giving deference to the bride, not the other way around. That doesn't mean she has to attend the reception, but it does mean she should offer to give up her invitation.

Some people deem things differently than others.

for instance, someone might see hanging out with their boyfriend's family as not the same as hanging out with friends aha. when in reality they don't live with ANY of them. so technically neither of those scenarios is correct.

so maybe she feels because the ceremony is short and she can be far apart in a mask, it is more comfortable. vs at a reception it is longer and people need to take masks off to eat. but again, as you said if anyone feels some sort of risk or uncomfortable at all, then why go? but everyone i guess has different ideas of comfort level.