I'm very sorry to here about your mother-in-law's multiple health conditions, and sorry that your family has to deal with so many unknowns and face the end of her life.
At least for now, blood transfusions at this pace (about weekly) wouldn't concern me. Transfusions can help her by supplying the oxygen she needs in her circulatory system, help her breathing, lessen fatigue, avoid headaches, and perhaps even help her ability to think and understand.
You don't want her to get more supportive transfusions than necessary, but the risk is low if they are infrequent and not over a prolonged period (which of course depends on whether the doctors think she has months left or years left). Asking the doctors to request "fresh" blood from the blood bank (recently donated, as opposed to near expiration) might increase the length of time between her transfusions.
If she gets regular transfusions for months, her blood may start to build up too much iron, which can harm organs if not removed by a process called chelation. You'd like to avoid that additional complication. You can read about it at the AAMDSIF website
The bottom line is that dealing with multiple health problems is a balancing act between treatments that may interact, and which each carry risks. But overall, transfusions may help both her quality of life and the length of her life.
I'm a caregiver, not a doctor, but I hope that I've given you information that will help you in your family's discussions with her doctors.