Hi momsh. Keep in mind that this is your occasion not theirs. The people you should invite are the people who are excited to be present on your family and babies life. It would be better if you choose Ninong and Ninang you know you can trust in helping you raise your child to be a good and loving person and not simply because you get along well. You really don't have to explain, just inform them that as much as you would love to invite more people, budget is tight and seats are limited so you're inviting only those whose presence really matter on the occasion. Remember that those who truly care will understand. :) ^
Tell them the truth. Dapat maintindihan kayo kung ano ang priority nyo.