I need to replace flooring in bathroom because of termite damage. My question is that my floor joist are wood I beams can I sister them with other lumber or how do I repair damaged areas? The entire joist is not damaged I am hoping I that only the flooring is only damaged.

Sistering Them Isn't Ideal But You Can, You Don't Have Very Much To Nail To On The Top And Bottom Sides Of The Joist,you May Just Use A 2x10 Nail It To The Side But Also Use A Jack Post For More Support

What happens when you put new material against old rotted material,is the rotten area will get into the new wood, then you have same problem all over again. Take and put jacks under the good ends cut out the rotten area, cut new joist at least 4 ft. on each side of cut out,nail good each side.You need to call your termite person to spray for termites .