How can i include my fiance's siblings?

My fiance has two teenage sisters who I am close to in a big sister kind of way. They are very eager to be included in the wedding somehow. I have a MOH and one other bridesmaid already and my fiance only has a best man. For a number of reasons it doesn't quite make sense to include them in the bridal party, however, I still want them to have a special part. I am thinking of having them walk down the aisle together in dresses similar to my bridesmaids but just take a seat in the front row and also asking if they would like to make speeches at the rehearsal dinner or the reception. I think giving them some sort of title would also mean a lot. I'm honestly not quite sure and very open to any suggestions on what we could do. This also raises the question of how to include my own teenage brothers, so any ideas on that would also be appreciated.