Here's the sentence in question:

"Sir Thomas Bertram had interest, which, from principle as well as pride—from a general wish of doing right, and a desire of seeing all that were connected with him in situations of respectability, he would have been glad to exert for the advantage of Lady Bertram’s sister; but her husband’s profession was such as no interest could reach; and before he had time to devise any other method of assisting them, an absolute breach between the sisters had taken place."

So Sir Thomas evidently wanted to help the Prices but was unable to use his interest/influence to advance Mr Price in the Marines. I knew that Army commissions could be purchased, and that the "interest" of someone with influence in the Navy was vital to get the early steps of naval promotion. This made me wonder if the phrase in bold just meant that Sir Thomas had no influence in the Navy, or whether there was something in the regulations governing promotion in the Marines which meant that Sir Thomas could not help. Frustrated by my failure to find the relevant regulations I wondered if anyone else knew the answer, that's all.