Doing a study and I’m trying to figure out the rule surrounding Yibbum, the marriage of an older brother to the widowed sister in law. I can’t find any mention of the older brothers marital status having sway one way or another. Also This study is in the traditional sense, so I’m not looking so much at the current thoughts on it, but what was customary during Roman rule?
Yes Yibbum is permissible when the surviving brother is married.
The oldest brother is the one who the Torah prefers to do Yibum. (Yevomos 24A) His marital status doesn't seem to have been a consideration. Particularly since according to Torah law there is no issue with having more than one wife. The Mishnah and Gemara discusses plenty of (mostly hypothetical) cases of married men doing Yibbum.
As far as what was actually done under the Roman rule: When discussing a Mitzvah that hasn't widely been in practice for centuries it is difficult to know for certain all the Rabbinic considerations used before practical real life applications . To answer the question however, it probably depended on how agreeable people were in general to the concept of having multiple wives in that particular location.
Based on a Posuk, the Gemara (Yevamos 44A) says, that if there was (e.g.) a large age gap between the widow and the brother, Beis Din would try to convince them not to do Yibbum and end up with an unhappy marriage. It's safe to assume they would have done the same in a place where multiple wives didn't work out.
Note: Although the reasons for not doing Yibbum today and the reasons for not having more than one wife today, are not the same, generally speaking the communities that had a more positive attitude towards multiple wives were also much later to stop doing Yibbum in favor of Chalitza.