My sister is my MOH. I hadn't thought about her husband doing anything during the ceremony - I figured he'd sit with his mom and dad, or maybe with the husbands of my other bridemaids - but my mother asked if I would think about a way to include him. She compared it to my having my FSIL in my bridal party, saying that I didn't know her well but was willing to include her because she is a part of FI's family.
To me, that comparison isn't valid at all. My FI grew up living with, playing with, and babysitting his half sister. My BIL, who don't get me wrong I think is a cool person, married into the family a few years ago. I don't think there's any expectation there that he "should" be included in the wedding party.
My FI is having only a best man - his younger half brother - and no groomsmen. So I don't feel comfortable asking him to include my BIL in his party. They hung out one afternoon while my sister and I went dress shopping, and that's it.
However, if someone has a suggestion about somthing BIL can do that would include him as part of the family, I'd be open to it. My initial thought was that he could be an usher and maybe walk with my sister until she reached halfway down the aisle and then go sit down, but then I read that the MOH is always supposed to walk alone?