What should I do about my slow sister-in-law who wants to run me?

My husband and I have been competitive distance runners for the past two years. We love competing for personal bests in formal timed races we enter, and it’s been a great activity we can do together that has helped us stay healthy. A few months ago, my sister-in-law took a sudden interest in running and has wedged herself into our running habit. We assumed that her interest would go the way of her previous fitness interests and fizzle out over time. (She went to Crossfit five times a week for two weeks, joined a spin gym for a month, tried aerial silks classes for a month, and then attempted P90X before selling all the DVDs on Craigslist after about three weeks.)

While we’re thrilled she’s trying to find something that works for her, having her along while we’re trying to run races has been very taxing. She is extremely slow and hasn’t seemed to grasp that running is not a team sport, and is a personal best endeavor. She gets upset if we post on social media about a race and didn’t invite her (we do about one per month), and when we do invite her, she throws a fit if we don’t stay with her for the race. During the last race, my husband got so frustrated and so concerned about his overall qualifying time for an upcoming race that he lost his temper and ran ahead when she stopped to rest for the fifth time, after which she wrote a nasty post on social media about how family stays with family and how she was “betrayed today.”

We have a race coming up in a few weeks, and it’s a big one for us, but we’re terrified to say anything about it online for fear that she’ll invite herself and then demand we run slowly with her. Should we just acknowledge that we’re going to hurt her feelings in the short term and wait for her to move on to something else? Is there something else we can say to her that won’t cause her to feel so hurt?