Jil,
Your mother is lucky to have you looking out for her and learning how best to help her. I'm not surprised that they don't want to use aggressive MDS treatment for an 88-year-old with other health conditions, especially since she hasn't needed transfusions so far.
I hope you can tell what your mother's real preference is about accepting some physical strain to get to see the whole family. Try to encourage her to tell you how she feels day to day, even if she's the type of parent who doesn't like to complain. It's a good sign that the doctor said OK to the trip, especially since doctors can tend to be overly conservative (protecting health status rather than considering the bigger quality of life picture). But it also depends on whether she feels up to it. Her cold was probably harder on her than her depressed blood counts have been.
If the antibiotics are doing their job then it's nice to have the choice of traveling. Chances are that your mother's ability to travel will lessen over time. If the doctor starts recommending against it at some point, chances are you and your mother will agree.
If you take this trip as planned,
here is a thread with travel tips, including remembering hydration. A little advance planning can make traveling safer. On a long drive you'll have to be flexible, willing to stop driving and take a break if your mother needs to rest. You're worried that it be the last chance for your mother to see your brothers and sister, but you never know -- part of being an MDS patient is living with a lot of uncertainty. I hope your siblings plan to make trips now and then to visit her too. It's a lot easier to receive visitors than to travel.