I want to ask my married sister to be my maid of honour. Can I include my brother in law in my bridal party so that they are both standing on my side?
YES, you can! Although most wedding parties have female bridal attendants and male groomsmen, it doesn’t have to be the case. You are more than welcome to have your married sister be your maid of honour and have her husband be your bridal attendant (vs. calling him a bridesmaid). The only thing you would need to consider is whether or not having a married person in your wedding party is culturally acceptable (ie. in some cultures, only single people can be in the wedding party. Check this out with your family if this is a concern for you.)
Having said that, you will need to make a few modifications to how your wedding will proceed. For instance, the bridal party will usually go shopping with the bride for her gown and accessories. You may/not want your brother-in-law to be part of the gown finding process.
You also don’t have to follow the traditional procession down the aisle. Each attendant can walk on his/her own.
Likewise, with a male attendant on your side, your brother-in-law will obviously not be wearing the same attire as your sister but you can certainly have some continuity in the look (ie. matching flowers, etc.).
Most importantly, inform your wedding photographer so he/she doesn’t stand around expecting to see the traditional grouping of attendants.