I'm not a lawyer but I have a legal mind. I also have a background in psychology but I'm retired now. So I always look for possible motives especially at the beginning of a case.
Basically the main motives are love, lust, loathing, or loot. But this author lists 15 motives for murder.
Why Do People Kill? 15 Motives for Murder, BRYN DONOVAN
In this case, I guessed it could be loot and excluded religious differences early.
When I read that this couple are separated, but still amicable, I guessed the divorce might not have reached the legal stage yet so that papers were not signed. So EP, not wanting to lose her lovely house, IMO could have tried to poison her husband first. When that didn't work, she made sure she kept a close relationship with the in laws. I suspect that the mother in law's sister and her husband did not have any children so they were going to leave everything to SP in their will and EP knew that. So if his parents and his parents sister and her husband died, EP would inherit all of his mother's sister's and her husband's estate and one-fourth of his parents' estate as they had four children. If SP had been there, she would have also received the house in her name as well. But as SP wasn't there at the lunch, she went ahead with it and included the auntie and her husband as SP was likely to inherit more money from them assuming they didn't have any children.
EP had to do this before any property settlement was agreed to in writing because she would then have hoped to at least get half of anything SP inherited plus half of anything they owned together and possibly more as she had been a stay at home mother. She would then possibly have had enough money to buy her estranged husband's share of their house.
If this theory is true, then a lot of thought and planning must have gone into this over more than a year and if she is found guilty, then she could be charged with premeditated murder when she thought she had got rid of any evidence and would go scot-free!
All in IMO