oh I am sorry to hear about your FIL.
If you are asked to do it, you just will I think. I read at my Grandpa's funeral. was crying buckets up to the second before and wondering how on earth I was going to get up there and get through it and I just did.
if you're crying before you have to read: take loads of tissues as you can't speak properly with a snotty / teary nose so you can have a good blow before you get up there. take deep slow breaths before. also take a small bottle of water in your bag and have a couple of sips before you get up there, just to get rid of all the snot and tears and clear your throat.
if you think you're going to cry when you're up there, slow down. swallow and look into the distance / over everyone's heads. if you see other people crying it will probably set you off too.
practise it before and perhaps put a couple of marks in for pauses / breath taking.
if you really think you may lose it, tell the vicar / person doing the service and ask them to be ready to step in for you.
if you cry during it no-one will mind, they really won't. but if you do start sobbing and losing it, best to step aside than choke on through perhaps.
you will be fine. people just are - don't know how really. and if you think you can do it, it might be nice for you to volunteer to do so if others are reluctant. difficult if you are spouse or child to get up there. good luck.