oooo this is a good question. Like @calmwolf mentioned above, I think a lot of it depends on your world and your pack structure.
Theoretically, if your main character shows leadership qualities, that is a great first step. But there’s more than that in an Alpha. You’re looking at a person who’s main purpose in the pack is to both protect, but also help keep the pack in line. Your main character would have to show these qualities - i.e., protecting the children, helping out the elders, mediating between warring packmates, etc. all the while also being able to hold their own amongst the other dominant pack members.
The hardest part of having a non-werewolf be Alpha, in my opinion, would be molding your story and your pack so that the other dominant members respect the non-wolf. Unfortunately, humans (maybe not so much wolves, but definitely humans) see “others” as outsiders. Which means your main character will have to struggle over that hurdle before becoming Alpha.
What you can do, and what may go a long way, is by getting the already respected members of this pack to trust and respect your main character. You’ll have to find a way to get the higher ups in the pack to trust that your main character can keep their pack, their family, safe. Combine that with the Alpha having no living children, and I’d think readers would think it logical that your MC is Alpha.
I hope this helps and is not at all confusing xD