Wow, how quickly people seem to have jumped to subjective assumptions about the O.P's character based on a few tidbits of information. She's 'not a nice person' etc. My God. You really need a bullet proof vest before posting something that leaves you open and vulnerable here sometimes!
Could it be OP, that you've been feeling undermined in terms of full acceptance within the family for some time and this event, no matter how trivial or even acceptable it is, has been the last snap of the thread so to speak? It must be painful to love your husband and desperately want to be on equal footing with his family, and yet feel like little more than a fond outsider. To make it worse, you are constantly aware that someone else was considered more family/liked than you. That has got to grate. I understand your 'Nigerian' reference comment as meaning that they identify more with the ex, because they have a shared culture, and this adds to your feeling of exclusion. I would find this hard to deal with after a prolonged length of time, more so after marriage and a child.
Other posters may say that it's good to show love to an ex and grandchild, and that's fine. But here it seems that they've gone beyond this, and are actively demonstrating not just continued love and loyalty, but preference regardless of, and in complete passivity to your feelings. I agree with a poster who said everyone should know their place. Nostalgia is not necessarily conducive to moving forward or accepting reality. Which may be the problem here.
Whilst you are completely wrong to make this an issue at such a sensitive time, I can understand your feelings on the fact that she was asked to read, but you were not asked at all. Even though he is grieving, I cannot imagine how he could think it best for his ex to give a reading and his wife not. Fine they were together 10 years, so she may know your MIL better, but surely space could have been made for you to speak also? Had I been the ex, I would not be comfortable with that arrangement. It sounds like she was only too happy to still be considered an essential family member above you. I imagine you are actually also grieving, albeit to a lesser degree; this is your child's mother. Had I been attending such a funeral, it is something I would have remarked on as a bit strange, and it would tell me everything about the family dynamics.
But due to the sensitivity of the timing, I do not think you should try and tackle this now. You will not get a good reception from your DH. I would leave it a few months, and then confront the issue, but whatever you do, do not blame the ex, or make it appear that you are jealous. It will not go down well. You have to also accept that your husband may always be tied in at least a small way to his ex. But there are boundaries and clearly you are not happy with where these lie. I think you have a right to more support from your DH.
FWIW. I think you are a nice person and sometimes, under duress,

not, just like most people, including myself