This is just a really odd situation, Imo. I try to imagine any number of scenarios. All below is speculation.
1) She served up beef wellington with deaths cap mushies inside them but didn't partake herself - Sus even if she argues she didn't know the mushies were dangerous when she picked them. A lot would rest on the credibility of any reason given for why she didn't partake - ie vegetarian? What did she have instead? If not vegetarian then it just looks incredibly suspicious that she didn't partake. And even if she is known to be vegetarian it still raises questions.
2) She did partake of beef wellington and didn't get sick. That looks even more suspicious. Too much of a coincidence to suggest that her own beef wellington serving somehow didn't have any mushroom in it. Or that maybe she gathered two sorts of mushrooms and she got lucky in that her serving had the okay ones whilst the other four diners all unluckily got wellingtons with death caps.
3) The deadly mushrooms were store bought. Not feasible. More people who bought and consumed from the death cap batch (tainted perhaps somehow accidentally with deathcaps?) would have reported ill by now. OTOH she may well be able to prove she bought store mushrooms for the meal, say, the day before and children may be able to corroborate they saw her preparing the meal with such that morning or previous day. But this still doesn't help her case. Assuming the fact stands (official tox report still outstanding) that the four relatives were poisoned by toxins found in death caps, then it just opens the door to the probability of her adding either fresh death caps (or dried/powdered death caps) to the wellingtons at some later point. And again, if she tainted all but her own portion and ate hers with the store bought only, there is no innocent explanation for her not being ill.
The only possible semi-unsus scenario I can come up with is that she ate wellington along with the others but didn't partake of mushroom sauce. She decided to make the sauce at last minute and as she had used up all the store mushrooms she used some dried/powdered mushrooms she had gathered from some time prior. She didn't partake of the sauce. This seems semi -credible.
Moo