Given the nature of the developmental concerns, they will be able to benefit from therapy. Those developmental problems will not interfere with therapy.
The delays with my 7 yr old is mostly with reading and fine motor skills everywhere else he is normal.
My 5 yr olds delays are with speech and fine motor skills.
It's imporatnt that you focus on your children, not on what might occur with the other child. In addition, you need some help as you deal with this. I can understand why you are not satisfied with one treatment session for your sons over a span of three months. Be assertive and be a strong advocate for your children. In addition, seek help for yourself. And your own therapist might be able to help you arrange a more satisfactory treatment for the children. Now, a note of caution: if the boys are developmentally delayed, depending on the degree of delay, it could be that therapy per se will not be particularlly useful. I really can't evaluate that from my vantage point.