Based on that: There is nothing wrong with you sharing the purchase of the cow with your relatives, so that you will have one seventh of the cow, which you will intend as an udhiyah – but less than one seventh will not be acceptable as a sacrifice on your part – and they will dispose of the rest of it in whatever manner they like, whether for a wedding feast or otherwise. But we should point out that the minimum age of a cow for it to be acceptable as a sacrifice is two years; anything younger than that will not be acceptable, even if it has a lot of meat. Please see question no. 41899. And Allah knows best.